How to experience Karma?

It is said that Someone needs to experience the truth to understand the truth.

Not necessarily, there's something called logical inference. If you see someone being stabbed in the groin, by just witnessing the painful expression on their face is enough for you to infer about the excruciating pain they're going through. You'd never ask the attacker to also stab you in the groin in order for you to "understand the truth", would you? Similarly for the Buddha's teaching. The thing with Kamma is that until one's already at a high level of attainment and possessed those supernormal powers like being able to see previous lives, there's no way to 100% see exactly the working of Kamma. That doesn't mean one cannot logically infer about its direction. Also, the Buddha's teaching in the Safe Bet Sutta offers some very useful and pragmatic insight into why one should spend some time contemplating on His teaching about Kamma.

Try get a better grasp on how the word is used in the pali Sutta;

Intention (pali Cetana), I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect.

"And what is the diversity in kamma? There is kamma to be experienced in hell, kamma to be experienced in the realm of common animals, kamma to be experienced in the realm of the hungry shades, kamma to be experienced in the human world, kamma to be experienced in the world of the devas. This is called the diversity in kamma.

"And what is the result of kamma? The result of kamma is of three sorts, I tell you: that which arises right here & now, that which arises later [in this lifetime], and that which arises following that. This is called the result of kamma.

And what are fabrications? These six classes of intention (cetana)

Here Sutta method of expression is that what is Called Kamma is also called Intention and what is called intention shares meaning also with word Fabrications sankhara.

Reasoning is that they are comorbid, conjoined coexistant and tied up. It's somewhat like the elements of both mass and speed are known & inferable from the one notion of 'a moving object'.

When you have a moving body, you know there is speed element and a mass element. Here they arent the same but one can't take speed out of the moving object as to separare mass from speed but they are different elements with clear difference in how we think about when seeing them with intellect.

Mind is different to a moving object, for one It arises as one thing ceases as another

Contact, reverends, is one end. The origin of contact is the second end. The cessation of contact is the middle.

“The past, reverends, is one end. The future is the second end. The present is the middle. And craving is the seamstress …

When further it is expressed;

"And what is the cause by which kamma comes into play? Contact is the cause by which kamma comes into play.

Contact is also a cause for dependent origination of other elements one can think about as conjoined with sankhara or think about in general, it's dependent origination all classes of the perceived & conceived. https://www.accesstoinsight.org/tipitaka/an/an06/an06.063.than.html#part-5

What one experience that one knows, what one knows that one cognizes, what one cognizes that one perceives, what one perceives that one senses, what one senses that is brought about [fabricated, sankhata],what is fabricated is conceived, what is conceived is inacted [verb conceiving, ie past conceiving is done, performed, inacted; is, will be or has been inacted]; what is inacted (kamma) is intended(cetana)

Is my method of expressing it.

To put it very simply, karma [sanskrit] (kamma[pali]) is usually translated as action, a sort of cause-effect relation. Therefore, if you subscribe to the view that all things that happen in the world happen due to causes and conditions, then you are subsricbing to karma. If, on the other hand, you subsrcibe to the view that there are things in the world that do not happen because of any cause or condition (that is, nothing is responsible for their coming into being and neither are they resonsible in producing any effect) then, there is no karma.

As per Buddha and very important BUddhist scholars, we are a collection of phenomenon ourselves (the 5 aggregates). Each aggregate is a result of causes and conditions being satisfied and their coming together is a cause for the individual phenomenon. One is thus a phenomenon (dharma[sanskrit] dhamma[pali]) oneself and therefore, like all phenomenon, a karma- arising due to causes and producing effects. To experience karma, is in this way, akin to experiencing the phenomenon fo aggregates. Or simply, one's life is nothing but fruition of accumulated karma.

Since we are ourseleves, like every other dharma, a truth; to experience the phenomenon (mind-body, from grossest to subtlest) is to experience the truth, knowing the truth (likewise from boradest to the deepest) experientially.

Yes. It's true that not everything is caused by karma. Some things are caused by other reasons, like the weather. The Sivaka Sutta talks about this. There's a summary at the bottom:

Bile, phlegm, and also wind,
Imbalance and climate too,
Carelessness and assault,
With kamma result as the eighth.

The Acintita Sutta states that you can't work out karma and its results precisely:

"The [precise working out of the] results of kamma is an unconjecturable that is not to be conjectured about, that would bring madness & vexation to anyone who conjectured about it.

What's more skillful is to develop one's virtue (sila), concentration (samadhi) and wisdom (pañña) as stated in the Lonaphala Sutta. That will mitigate the ill effects of past karma.