Very engaging question, Leigh. I look forward to seeing all the responses to this. To be quite honest, I have no idea what 'makes' me 'feel' like a girl. I know I love being a girl, yet I can't really pinpoint the why of it... it just feels right...whole...proper...me.
How do very young children know? Last year I read As Nature Made Him, about the boy whose penis was severed in a botched circumcision. His parents raised him as a girl, and continued to raise his twin brother as a boy. So how did this kid continually defy what his parents told him, and what society told him, from a very young age? He just acted like a boy, regardless of what his family said and how they dressed him. He knew deep within himself that he was not a girl.
So I would take that to mean that perhpas what we are cognizant of as adults is only secondary to what we instinctually know as children.
Valerie