You never know until you ask
Men who have kinky, perverted, SM sex with other guys are a diverse group, and they include female-to-male transguys. FTMs, transmen, transsexual men, transgendered guys, and trannyfags (language can vary by individual), are a vibrant, hot, sexy, and active part of the men’s kink community. They come in all shapes, sizes, perversions, kinks, sexual orientations, and attitudes.
Finding out that the hot guy you are cruising at that play party is trans doesn’t tell you anything about what he is into, how he relates to his body, whether he’s a Top or bottom, what kinds of surgery he has or hasn’t had, what hormones he is or isn’t taking. Don’t assume you’d just “know” if a guy was trans. You won’t. If it matters to your play, then ask as part of your negotiations.
There are no particular rules except respect. There are no rules about when (or even if) a guy should tell you he is trans. Playing with a trans guy is just that – playing with a guy. And, just like with other things, some FTMs don’t care if others know they are trans while others consider that information personal and confidential. Before you “out” someone, you might want to find out how he would like it handled because disclosure raises issues of safety and possible discrimination, both within the SM community and without.
If you have questions, educate yourself– go to a panel, read a book, check out a resource group. While some guys don’t mind doing a little education now and then, if you find out that hot leatherman at the bar is a transman, he’s probably more interested in what your kink is than in giving you a Trans 101 lesson.