I agree, Joe, that the only way to know is to watch a person's eyes or face. I find outward signs typically to be misleading. Plenty of people wear certain clothing or have a certain relationship dynamic, completely unrelated to BDSM.
I can't even say what it is about some people that makes me pause to wonder. I tend to be on the lookout for dominant guys rather than subs or women. There is one guy where I work now who has caught my attention--he's not in a position of power, but I can tell that he enjoys responsibility and leadership. He's comfortable with himself and with asking for what he wants, but he's also very courteous and respectful, even to lowly admins like me. :-) I can see him ordering a woman to her knees, taking what is his, then giving her a nice bath.
More than wondering about other people, I wonder if they can tell about *me*. I'm 100% taken, but the idea of others (especially male doms) having that certain feeling about me is positively delicious!