I was fooled once. I've always been a sucker for a woman in a red dress, and this was a very fetching person out front of a Soi 6 bar that I had thought only hired those with an honest-to-gawd vagina. I invited this looker inside and bought us each a drink, then I very politely asked "man or woman?" "Man" was the reply. Again, I politely declined any further P4P talk, but we spent a few minutes chatting, then we each went our separate way, no one the worse for the interaction.
As many have said here, just ask them. Be polite, and you don't even need to buy them a drink first.