if your friend who knows you're trans and you talk to her about it, why don't you ask her if this other girl knows? (i assume you work with both of them, that's what i got from this). either way, even if that girl doesn't know or you don't wanna use your friend to find out..you can still have a convo and find out where this girl stands on LGBT issues if you haven't before. that usually gives you a good indicator on how they might react if you come out to her. bring up a news article you saw or something you saw online, w/e. something casual and if she is cool with it she just might think you're an ally at most (unless she did find out then either way it won't matter) right?
if she isn't cool with it - move on. if she is, test the waters then. you can go on a date (or even a few dates) and not tell her right away. you don't have to disclose your trans status to anyone and you decide when you do. generally people do it on a first date, or the first few. especially if it's gonna go somewhere. honestly being pre-t isn't a deal breaker (although it might make us dysphoric). if she's cool with it all, she might want to know how far along you are, ask questions, etc. the whole dating thing can be tricky (whether you're pre everything or not) but when i went to talk to people about how to go and disclose it..they just told me to do it as you do when telling someone about yourself on a date kind of thing like when you're finding out about someones interests and w/e.
either way it's all a big change and you'll get into the groove of things once you take that first step. you'll figure out when to tell someone, how, etc. just try not to be discouraged if she isn't cool about it and don't be petty about it either if you decide to tell her. learn from it and move on because that's one girl and for that one, you'll find 5 who are cool with it. especially since you're a bit older, people are more open minded around your age and understanding and probably even informed on it.
