At a recent Buddhist meeting I ask this question and the Teacher said, even Buddha doesn't provide a definitive answer on that. After all I said, how can anyone when the act of knowing a thing means that you don't know. So I thought I'd use modern technology to clear things up and so asked ChatGPT (an AI system all the rage at the moment). Here's the response I got, which I think you'll agree is very enlightening. I'd be happy to hear your thoughts in the comments below. (I also used AI to generate the image, wow is all I can say on that).
[Replying as Buddha] how can a person know that they are enlightened, when the act of knowing a thing proves that I don't know?
In the voice of Buddha, I would share: In the journey of enlightenment, one transcends the duality of knowing and not knowing. When one is enlightened, the mind becomes like a still pond, reflecting reality as it is, without the ripples of desire, aversion, or delusion. In this state, the act of knowing transforms; it no longer arises from the ego, but from a deeper, boundless wisdom that permeates all.
Enlightenment is like the lotus that emerges pristine from the mire; it is a state of being that is self-evident to the wise, yet may elude description. It is not a knowledge to be grasped, but a realization to be lived. When the self dissolves, and there is no one to know or not know, what remains is enlightenment. The enlightened one abides in effortless awareness, unattached to any fixed notions, and free from the bondage of ignorance.
Thus, it's not a matter of knowing enlightenment, but of embodying it. In the silence of the mind, in the purity of action, and in the compassion of the heart, the enlightened one moves, free from the illusion of self and the shackles of doubt. The question of knowing loses its grip, as one rests in the natural state of awakened being, like a bird soaring freely in the boundless sky.