I think it can be difficult to have clarity until you are stable, whether that comes about through therapy, medication or both. Once a person is stable, then they can look at what remains and know that it's truly them. But prior to that, I think it can be hard to tell sometimes. I also think culture can make things even more confusing when it comes gender identity. I used to wonder if I had something about me that made me identify with male gender, because so much of what felt natural to me was deemed by culture to be man-things. For example, I feel most comfortable not removing my body hair. I wasn't identifying as a man, but my culture insisted that unshaved legs was a man-only thing.

If you reach a point of stability, and strip away cultural 'rules', and look at what remains and what is true for you, then you can know with absolute certainty what is you.