How are good and bad Karmas determined in Buddhism?

Good karma are intention which drive actions which mature into results which are deemed favorable and bad karma are intention which drive action which mature into results which are deemed unfavorable. Karma is the product of intentional action may it be mental, verbal or physical.

"how do Buddhists determine what actions are good Karmas and what actions are bad Karmas?"

Only Buddhas can determine what actions are good karma and what actions are bad karma.

Buddhas determine good and bad karma by looking at the past and present karma of beings. By knowing the karma of beings, they know what actions would propel those beings towards liberation, and these actions are then termed "wholesome".

"Do Buddhist scriptures discuss how to determine this?"

No. Explaining to a human being how to look at the past and present karma of beings would lead to madness. It is too complicated for us to comprehend this madness. Only Buddhas can comprehend it.

"what's required to get positive experiences through the law of Karma.?"

Read the suttas where the Buddha teaches what is required to get positive experiences.

"do Buddhists simply classify Karmas into good or bad, or do they subdivide these categories further?"

Buddhist classify karma into good, bad, neutral.

Please let me offer one other alternative: karma is not good or bad - it just is.

We have watched centuries unfold where we are told that this action is bad or that action is bad from pretty much a moral perspective. I would contend that morality has nothing to do with karma. Morality is a cultural concept whereas karma is a part of science and nature. The only "morality" rule in nature and science is the golden rule and this happens as empathy resulting from the byproduct of a communication tool developed in our frontal cortex. All other morality rules come from this or are merely attempts to control.

Now, that said, it is true that bad acts - those frowned upon by society - do create more "bad karma" the same way that acts we perform that the public likes create more "good karma". Crying creates sadness which creates depression while smiling creates happiness which leads to good feelings. But an act we could consider bad at one time would be good at another.

Karma is little more than the cosmic momentum created when we say or think or perceive or do something. It alters matter and perceptions and body reactions and the like and this results in a mountain of consequential effects. It's delineation as good or bad depends on the context and "experiencer".

Please see this answer on karma, rebirth and effects of karma on this life and future lives.

From the Nibbedhika Sutta:

"Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect.

In Buddhism, intention is the basis for karma. There are some guidelines on removing unskillful thoughts and qualities. The story of the blind elder monk Ven. Chakkhupala in Dhammapada 1 and the story of the hunter's wife in Dhammapada 124 further illustrates this.

Dhammapada 1: All mental phenomena have mind as their forerunner; they have mind as their chief; they are mind-made. If one speaks or acts with an evil mind, 'dukkha' (suffering) follows him just as the wheel follows the hoofprint of the ox that draws the cart.

Dhammapada 124: If there is no wound on the hand, one may handle poison; poison does not affect one who has no wound; there can be no evil for one who has no evil intention.

It appears that the simplest way to identify unskillful thoughts and actions, is whether they are imbued with the three poisons or not. And for skillful thoughts and actions, it's the opposite.

In the Vitakkasanthana Sutta, we find that unskillful thoughts imbued with desire/ greed, aversion/ hatred, or delusion have to be removed.

In the Vina Sutta:

"Monks, in whatever monk or nun there arises desire, passion, aversion, delusion, or mental resistance with regard to forms cognizable via the eye, he/she should hold the mind in check. [Thinking,] 'It's dangerous & dubious, that path, thorny & overgrown, a miserable path, a devious path, impenetrable. It's a path followed by people of no integrity, not a path followed by people of integrity. It's not worthy of you,' he/she should hold the mind in check with regard to forms cognizable via the eye.

And the same applies to the other senses.

You can find a similar discussion in the Aparihani Sutta where it talks about removing "evil, unskillful qualities such as greed or distress".

In the Sacitta Sutta, also teaching us to remove unskillful thoughts:

"If, on examination, a monk knows, 'I usually remain covetous, with thoughts of ill will, overcome by sloth & drowsiness, restless, uncertain, angry, with soiled thoughts, with my body aroused, lazy, or unconcentrated,' then he should put forth extra desire, effort, diligence, endeavor, relentlessness, mindfulness, & alertness for the abandoning of those very same evil, unskillful qualities.

Detailed classification can be found in the Abhidhamma in the volume called Dhammasangani ("Enumeration of Phenomena"). We can find a classification list in the work The Abhidhamma in Practice by N.K.G. Mendis:

Sa"nkhaaraa is a collective term for the other fifty cetasikas (mental factors). These fall into four groups:

• Universal mental factors (sabba citta saadhaaranaa)
• Particular mental factors (paki.n.nakaa)
• Unwholesome mental factors (akusalaa)
• Beautiful mental factors (sobhanaa)

You can read the details on this page for the full classification of the 50 cetasikas.

There are also other classifications there like the different types of cittas (momentary states of mind and consciousness) - vipaaka, kusala, akusala, kiriya. They have association with roots (hetu, mula - see below) and feelings (vedana) etc.

There are six roots. Three are kammically unwholesome (akusala); the other three may be either kammically wholesome (kusala) or indeterminate (abyaa-kata), depending on the type of consciousness they arise in. The unwholesome roots are greed (lobha), hatred (dosa), and delusion (moha). The three roots which are wholesome in some cittas and indeterminate in others are greedlessness (alobha), hatelessness (adosa), and undeludedness (amoha). Though these last three roots are expressed negatively they have positive manifestations. Greedlessness manifests as generosity and renunciation, hatelessness as loving-kindness, and undeludedness as wisdom or understanding.

The above are only the Theravada texts. There will undoubtedly be additional discussion on this, in the Mahayana texts.

You can find useful info also in Ven. Thanissaro's Kamma: A Study Guide.

OP: I think skillfull vs. unskillful has to do with what will lead you to liberation vs. what will serve to keep you in Samsara. But my question is about what will serve to give you positive experience in Samsara vs. what will serve to give you negative experiences in Samsara.

Skillful thoughts and actions will give you a positive experience in Samsara and could also lead you to liberation. Unskillful thoughts and actions will give you a negative experience in Samsara.

But liberation requires much more than skillful thoughts and actions. It also requires one to abandon all the fetters.

For e.g. by practising one or more of the Brahmaviharas (loving kindness, compassion, empathetic joy and equanimity) which is skillful, one could obtain the pleasant experience of birth in the heavans. But this is not permanent.

That is, there are certain actions which will lead a person to have a positive experience in future, either in this life or the next,

The Pali suttas do not literally state this. The Pali suttas (eg. AN 6.63) state there are three results of kamma: (i) now; (ii) later; (iii) later again.

For example, the kamma of drinking alcohol has three results: (i) pleasure now; (ii) headache tomorrow; (iii) habit, craving or addiction later.

Next life kamma is an interpretation rather than a literal reading of the Pali suttas. In the Pali suttas, words such as 'birth' ('jati'), 'death' ('marana'), 'body'('kaya'; group) do not necessarily have a physical meaning.

The Buddhist commentaries & suttas (somewhere) state how the ordinary person views Dhamma is the opposite to how an enlightened person views the same Dhamma teachings. In the commentaries, this is known as the 'two truths'. The ordinary person reads everything as physical materialism where as the enlightened person reads everything as it really is. Thus, the Buddha does not speak lies when he uses language that can be interpreted in two-ways.

In is not correct to assert Buddhism teaches about past & futures lives because this is not literally said in the original scriptures (although is literally asserted in later scriptures & commentaries).

And Hindus and Buddhists also agree that ultimately the goal of life is not to just do good Karmas so you can have positive experiences in future, but rather to break out of he whole cycle of Karmas and their consequences. Buddhists believe that this can be done through the Eightfold Path, and Hindus believe in other means like knowledge of the supreme being.

The Buddhist idea seemed to exist systematically before the Hindu idea. At least the idea of moksha with Brahma appears to be not mentioned in the Pali suttas, which report discussions with Brahmans who followed the Vedas. While the early Upanishads are not mentioned in the Pali suttas, it appears the ideas in them about moksha seem to be either immature or not widespread at the time of the Buddha.

But my question is about what determines good Karmas and bad Karmas. Hindus believe that the Vedas (and texts derived from them) tell us exactly what actions constitute good Karma and what actions constitute bad Karma. But Buddhists reject the Vedas.

The Buddha did not reject anything in the Vedas he agreed was beneficial. The Pali suttas explicitly state the Buddha was a friend of the Brahmans and wished no harm to the Brahmans. When the Brahmans tried to argue (like Jews) they were a chosen or superior caste, born from Brahma's mouth, the Buddha replied if they do bad kamma how can they be superior?; to which the Brahmans agreed. So both Buddha & the Brahmans had respect for good kamma as a determinative factor for a superior 'birth' (status).

so my question is, how do Buddhists determine what actions are good Karmas and what actions are bad Karmas? Do Buddhist scriptures discuss how to determine this?

In Buddhism, the ten skilful kamma are non-killing, non-stealing, non-sexual-naughtiness, honest speech, pleasant speech, cordial-non-divisive speech, beneficial speech, non-greed, non-hatred & non-delusion. Apart from this, there are many other lists, such as in the Sigalovada Sutta.

In Buddhism, everything is determined on the basis of causing suffering/harm & leading to freedom from suffering/harm, which can be known via personal experience. Refer to the Kalama Sutta.

Note that I don't think the answer is "The Eightfold Path", because that's about what's required to break the cycle of Karma and consequence,

Correct.

Also, on a side note, do Buddhists simply classify Karmas into good or bad, or do they subdivide these categories further?

Basically two-categories, namely, wholesome/skilful & unwholesome/unskilful (although there is a sutta with four types: white, black, white-black, neither-white-black).

Naturally, there are more refined superior good kammas, such as excellent generosity, meditative heavenly jhana, loving-kindness (metta), serving arahants, etc.

For example, there are many different godly realms in Buddhism, which are the results of different kammas as well as human, animal, ghost & hell birth from different kammas.

Godly realms represent special attainments, such as worldly wealth, political power or higher spiritual qualities, such as psychic powers.

Human birth means being moral.

Animal birth is being immoral, ghost birth is being chronically craving & hell birth is suffering & torture.

Because Hindus believe in a more fine-grained system where different kinds of good Karmas produce different kinds of positive experience.

Buddhism is an experiential tradition. The degrees of kamma are known in the heart.

Karma is intentional thought. If the intention is associated with greed, hatred and ignorance, it's bad Karma. If the intention is associated non-greed, non-hatred, non-ignorance it's good Karma.