I am in a similar situation right now, except I basically pass at this point, even with my voice.
Before we actually met up, I initially though she knew I was FTM, because she asked what gender pronouns I preferred. I said "he and him". This is a pretty common question in the area I'm in though, so I'm not entirely sure; she never asks me about my gender identity (people always have from my personal experience) and there are a couple of occasions where she talks to me as I'm a cis-male. Granted, my demeanor is pretty masculine, but I dunno.
I'm already past the point of hooking up with this girl and from what I can tell, she is really open-minded, but I'm still not sure if she knows and if she doesn't that telling her would drastically change what she thought of me. The worst part is that if she does already know bringing it up is kind of awkward for me, but she sends mixed signals, so I think I'm going to do have to just so there are no weird surprises. Yay.