When you're trans, nobody ever teaches you the ins and outs of using the bathroom. Most of us are a wizard at this by third grade. So as somebody who used to be a guy, here's what'cha gotta do.
Be as obnoxious in the bathroom as possible. Heckle any guy who's taking a dump. Make plenty of comments about the color and smell of your pee. Fart as loudly as possible, and make comments about that, too. Climb the stall walls. Flush the urinals repeatedly, just enough to fill them to the brim, but not enough to make them overflow. See how long you can maintain this waterline. And if there is a guy taking a dump when you leave, be sure to turn off the lights.
Follow these steps, and nobody will EVER question your gender in the bathroom.