I kind of have a hard time with this too, because yes, sometimes, but no sometimes.

In general I can tell who is or would be submissive, or not. But I have no ability to "sniff out" Doms... in fact I've been quite surprised a few times when somebody tells me they are.

First impressions are generally really bad for this. A few weeks ago I sat down with somebody in a kink setting and was thinking, based on a whole lot of things, that it was pretty obvious he was a sub. A little while later we ended up talking, and it was very very obvious I'd been incredibly wrong about that.

With people who are completely vanilla I can sometimes tell which way they would tend towards, and sometimes not. Some people make it obvious they want to be controlled, or are very deferential.

In a kink setting, it's far easier. Obviously experienced "players" will know for themselves, and there's just not much guessing or confusion. People who are new, or curious, though, may say one thing (and may even believe they have a certain orientation), and act completely differently. But because it's framed in a certain setting, it's easier to pick out those traits.

As to whether it happens in real life... I've been through it, once. I don't think it's exactly common though. It's not like Doms are out and about prowling clubs and back alleys, hunting for some vulnerable vanilla who doesn't know they're submissive yet, or anything like that.