Well, the safest thing if you're unsure is to only use "one-holers"-- single commode toilet with lockable doors. You often find these at fast food places and gas stations, for instance. In this case it probably won't matter to anyone which one you use. It wouldn't be a bad idea to scout out a few of these in advance of an outing just in case.
I've read dozens of versions of this thread, though and the consensus seems to be that if you are best off using the restroom designated for the gender you are presenting yourself as. If you're wearing makeup and a dress, you probably don't want to go to the men's room.
~ Lyric ~