[Pollster1]

They aren't saying, "Hey, sometimes I can guess who's gay and who isn't." They're asserting that their sense is a lot more infallible than that, and they don't.

Mostly I hear it in a context like when Liberace announced that he was gay. It's more of a "no kidding" than a "I can detect hidden thoughts".

[x]

It's not hard to claim you knew it all along after you find out.

But I heard this gaydar stuff when the Jared the Subway guy story came out and many people just "knew" that he was gay all along. Or you could tell it just by looking at him. Or they had friends who knew people who had friends who said "everybody" who went to college with the Subway guy just "knew" he was a "flamer."