There are legal provisions for people using a restroom, or any other public room for that matter, that engage in perverse lewd and/or lascivious behavior. If anybody, regardless of their gender identity or what clothes they happen to be wearing, engages in such behavior, there is recourse. If somebody is just being creepy in a restroom, again leaving out gender stuff, without actually crossing any legal lines then you have every right to get away from them.
Feeding into people's irrationality when it comes to who is in a restroom with them, and where trans-whatever people belong has become the conventional method for people/politicians with bad intentions for us as a group to shoehorn a debate for limiting our rights into their various platforms, and I think we should not be feeding into it. Use the bathroom you feel most comfortable in, then when you are done, leave. This isn't something to freak out about. If somebody is being weird in a bathroom, then they should be dealt with accordingly, regardless of their gender whatever.
My answer to the original question is if somebody is presenting as female, they are "she" until they tell me otherwise.