If a person reaches enlightenment, will she or he know it? And, if so, how will they know it? Will others?
This is a trick question . . .
Yes, each person will know (or sense) for him- or herself that things have changed, and one would sense a change in oneself. And it would be a very humbling experience. And the sense of self (i.e., the ego) will change.
We have to remember that full Enlightenment (Pari-Nirvana) will occur only after death. While one is alive, we can experience Enlightenment (reach Nirvana) in a limited way. One is still human, so one is still limited.
One does not know if one has reached Enlightenment, but it is more of a sensing that things have changed, and that that person has changed. There is no way to describe that experience; there are no words to express that experience; there is no lightbulb that suddenly goes on. There would be a sense that things have changed and, yet, nothing has changed.
Others will know, again, only by sensing it, feeling it, or somehow know to be in the presence of someone very special. The key characteristic would be a deep and profound humility. There is also a great sense of Oneness and peace.
But trying to describe something that is indescribable is fruitless. Each person is unique and thus each person’s Enlightenment is unique, which makes trying to describe this experience impossible.
Maybe, we should reverse it and remember that anyone saying “I am enlightened” is proof that they are not. As soon as that person says “I,” we know they have not let go of their ego.