How could the Buddha know that he had attained enlightenment when he didn't know what it was?

Imagine for a moment that you are walking around feeling tired and achy and miserable, and you don't why. Then you happen to look down and see that you're carrying a bowling ball. You must have picked it up the last time you went bowling (whenever that was) and forgotten to put it own, and it's been dragging on you all this time. So you put the bowling ball down, and the difference in how you feel is immediately tangible.

That's all that's happened here. The Buddha put down the weight of egoic existence, and the feeling was immediately tangible.

Having experienced a completely profound release from stress and dissatisfaction, he let the contemplations of past and present Kamma go, so as to turn to this immediate experience and how to share it with others, in such a way as to attain to it themselves.

The Four Noble Truths bear little relation to the account of this first night of Awakening, yet enabled the listeners to share the experience themselves.

In terms of Dependent Origination, the role of ignorance is particularly subtle. Unknowingly activities, conditionally arise, resulting in a consciousness that doesn't know of the arising of its own origination factors. Instead, it progresses to the unspoken, unquestioned projection of Subject and Object, Nama Rupa. Which in turn also knows nothing of its own arising. After stream-entry, a consciousness may arise that can discern this, but it too knows nothing of its own arising. The arahat just lets it all go.

Ever get drunk? How did you know you were drunk the first time it happened? Enlightenment isn’t like being drunk, but like that state, entrance into it is unmistakable. Upon reaching it, the scales fall away and the very experience of self and suffering is irrevocably and fundamentally altered. The Buddha knew full well that this, what he had discovered, was utterly different from anything he had learned up to that point. Enlightenment really does speak for itself.

I am citing my answer to another question - Question

Part of my Answer:

When the Buddha (and so many other Buddhas) talk about no-self, they come from a direct experience of unity with everything. That is the source of “no self”. Their perspective is abolished. There is only truth for them. Unlike our experience where we are this body and mind and everything else is separate. And we have opinions about everything from our perspective.

Ever know the difference between Gautama and Buddha? Gautama is a person. Like you and me. Buddha is a general term given to one that knows the truth of everything. Buddha is one who has attained self realization. So, there are many Buddhas.

So when Gautama attained to self realization he BECAME a Buddha. And when Gautama attains and becomes a Buddha, Gautama as a person is gone. There is only the knowing, Tathagatha. The knower takes a backseat. What we know as I, me, self (in this case Gautama) takes a backseat. So he just knows.

This article highlights an important point like so

The Buddha asked himself similar questions, and after some years of searching came to extremely ­subtle, profound, and liberating truths—and these were truths, not beliefs; they were self-evident, obvious actualities. The view of himself as an individual, as ‘this person ­Siddhartha Gautama’, dissolved when he saw beyond thoughts, and he woke up as if from a dream. In consequence, he knew freedom from the horrors of the dreamlike existence that many, perhaps most, of us are engaged in right now. His life then opened out into the immediate, the spontaneous, that which is beyond thoughts, beyond views and opinions, and beyond habits and shadows of the mind.

I only read the title first - and user17's answer. I really don't get the story in OP.

But the title question strikes me, and it reminds me so much of L. Wittgenstein in his Tractatus. He tries to logically prove that 1+1=2, and in the end says something like:

"Well somehow I deeply understand all this, but due do limitations of myself and of language I cannot explain in words.

May my words be helpful for you as a throw-away ladder."

Now in Buddha's case a biological (man/nature) and spiritual (moral) enlightenment is also involved. And before I get lost (or raise protests) because I chop up enlightenment, I stop.

How could the Buddha know that he had attained enlightenment when he didn't know what it was?

He felt it.

Where does this question or idea come from, if I may ask?

In the third watch of the night He had the knowledge of the exhaustion of cankers, knowing that He had eradicated all defilements and made an end of ill.

The Three Supreme Knowledges of Lord Buddha

So the Buddha knew he had overcome defilements and was enlightened due to knowledge of the exhaustion of cankers.

Also with the arising of knowledge of the exhaustion of cankers the Buddha knew what enlightenment was.

This is a great question. In Buddha's time, there were many other ascetics claimed that they have attained Nibbana. However, the definition of Nibbana was varied. Buddha realised that the elimination of Dukkha is the Nibbana. Then he defined the Dukkha with his revolutionary definition including Sankhara Dukka. He realised the cause of Dukkha is the craving. He also realise that the way to eliminate craving is to follow the Noble Eightfold Path. He also discovered the way to stop Dependent Origination' The night he enlightened he knew that he does not have any craving. This process is called the review.Paccavekkhana ñana:

https://en.wikipedia.org/wiki/Vipassan%C4%81-%C3%B1%C4%81%E1%B9%87a

He also the first person to realise not self-nature of our existence.