I never thought of myself as lesbian or butch but I was also never that much into women though I'm kind of pansexual, really, though I didn't figure that one out until I began to date my girlfriend. Before that I identified as male-attracted. To me, a big indicator was that I disliked marking myself as female when asked on say surveys, online sites and the like, and I'd stick with an unknown or other third variable whenever possible. Calling myself female felt wrong, period. It caused incredible psychological discomfort though I could never say why it did. It just didn't feel right. However, since I thought I was supposed to be female and label myself as such, I didn't think of the possibility that I could be male so I just went with unknown for I don't know how long.
What really triggered my move from unknown to male was when I really began to think about this and think deeply about how unknown had just been this cop-out for all these years. A cop-out of thinking I cannot be one thing because I'm told I cannot be, as opposed to how I really feel, while resenting the other option presented to me and who I was supposed to be. And most importantly, even though I wasn't trying very hard to be either male nor female in presentation though I did wear women's clothing at the time, I didn't feel happy with who I was. Female/other/queer wasn't who I really was. It's difficult to explain but I came to the conclusion that I should try out being male for a while, if I didn't like it, well I can always go back and then I've been there and done that and that's all there was to it. As it turns out, I didn't go back. I feel so much more happy with myself, who I am read as, treated as, how I present myself and how I can be overall than I ever did when I was female/other whatever. It just clicks.
Also, to go back to the butch thing, there's just a feeling... very difficult to describe or put into words, but just this feeling that butch women or women happy being women but wanting to venture into other kinds of female gender roles, just feel different from yourself, if you are a transman anyway. I'm friends with a woman who I would never classify as stereotype in any way possible because she's not, but despite that she's also very happy being a woman and the distinct difference between us is stark. Very difficult to put into words but I just know that our experiences differ. She's not unhappy with being a woman even though she doesn't feel like she wants to be womanly either, so to speak. And you notice this with butch women too. They may choose to look masculine but deep down they are women and they know it. Transmen don't have that, simply. They don't have the whole "I know I'm a woman on the inside and happy about it" going on.