Don't worry about it. If someone is trans, either they're gonna make it so that you'll call them the gender they prefer, either the problem is theirs to handle and a professional will be much more appropriate than any worry of yours, either they might be trans but don't WANT you to call them their preferred gender yet, since they're not presenting as such for now. Some trans people don't pass, true, but if a trans woman wants to be called a woman, no matter how her actual body looks or how her voice sounds, if she's wearing a dress, chances are that she wants to be called she.
Really, things are much more straightforward than one might think. As long as you address people the gender they appear to be presenting as, not the gender they appear to be born as or the gender you think they might secretly be inside, you're gonna be fine.