Sometimes I can, sometimes I can't. I find it harder with couples- for example, I met a girl at a party a while back and within 2 minutes of conversation had picked her (correctly) as a sub. But I met a couple recently who I'd never in a million years have thought were D/s had my Dom not introduced me to them.

So, I guess the moral is sometimes you can, and sometimes you can't. But it probably gets easier with practice.