I really don't think you can tell. I'm a very confident, assertive, and outgoing person everywhere but the bedroom, including in dating my relationships. Yet I've been strictly sub until quite recently. I know lots of people who are unassertive in day to day life but have absolutely no interest in bdsm, or even some who suddenly become dominant in the bedroom.

There are a couple people who have been able to spot it in me somehow, yet they were always subs themselves. I have no idea what they're seeing that tips them off, but it seems some people have a sort of intuition for it.

The only thing that possibly might give you a hint is how they flirt. If they flirt with you in a submissive way or in a way that suggests you are a dominant, it's probably a sign that they are submissive.