I wish I could answer this conclusively for you. All I can tell you is I am still fighting doubts of my own. They don't even make sense really, but I still cannot get rid of them. I don't feel like I was a "typical" transgender case. So much of what I read and heard was stories of people who felt things so much stronger then I ever did. Every time I wander around in the trans community I hear people talking about how it is they have to fight themselves to stop them from taking a knife to their privates. Its partly why I rarely do venture into these threads or spend time with others. I never liked the way I looked, but I never EVER had that level of certainty or self hatred about how I was then. When I was a little kid I didn't do much in the way of stereotypical girl stuff, I remember dressing as one, exactly once and never doing it again until after puberty. I also have doubts because for me my desires became a fetish, the hornier I got the more I wanted to be a girl, and when I eventually came, the desires were replaced with shame and disgust. The shame and disgust have faded completely in time but the question of does this mean my feelings are not real remains.
But I also know(and please dont take this as a recommendation, its not) when I do things that lower my inhibitions(MJ, Drinking) my doubts fade, and usually the further gone I am the less doubt I have. I know that I spent my whole life dreaming about what it would be like if I was a girl instead of a boy. I know I have never in my life really put in any effort in my male appearance, even as I thought it would cure my social ills, I could never put in the slightest effort to get in shape or dress nice... I also always wondered how much my fears of transitioning played into my doubts, I have seen first hand just how ugly people can be to trans folks AND their families. What I saw was a long time ago and not likely to occur today, but it still terrifies me to this day. I don't know about you but I can always find a million reasons not to do something I am scared of and none are "because I'm scared"