I can tell a gamer by mannerisms, and once made a friend simply by asking him rather cryptically. "Do you play?". Another time was when a couple of guys moved into my building, and I thought, "They must play", but held off on asking. A couple of months later I was leaving for work, and overheard one of these guys talking about his latest campaign on a cell phone. I butted in, and we got a short-lived campaign started, before they moved back out of state. Because I didn't trust my instincts, I missed out on a few months of what was looking to be one of the better campaigns I've ever run.
Trust your Game-dar.