I don't know if there're particular "signals", per se. I spent some time when I was younger seeking out people I thought had inclinations that way but hadn't pursued them. Corruption of the innocent is a strong motivator for me.

I would look for someone who would give in and follow my instructions even though I was, essentially, a complete stranger. You'd be surprised at how easy it is to have this sort of interaction with someone. It can be as simple as changing their product selection at the grocery store ("No, no, that one's no good; you should try this.").

Another thing I looked for was in a handshake. I changed my handshake so that instead of gripping with all four fingers, I'd grip with my pinkie and ring finger while sliding my fore and middle fingers up their palm to their wrist. I have rather long fingers, so this would typically place my fingertips at or near their pulse point. For some reason, this unexpected intimacy gets a strong reaction from someone with a submissive bent. (Try it in a business setting and you're likely to be looked at oddly, though.)

(This next bit's going to get me into trouble with someone, I can feel it coming ...)

It's been my observation, that submissives tend to, in general, share many of the traits of those who wind up in abusive relationships. So those are the traits to look for -- someone who's already (or likely to) self-identifying as a submissive, though, will typically have more self-respect and sense of self-worth.