Mine's pretty accurate, well, as far as I know!
I agree that is is often in the way someone looks at you, holding your gaze for an extra nanosecond. It's just like the way you can tell the difference between a guy who is friendly and a guy who is attracted to you even if he says nothing obvious.
Even when someone is not necessarily attracted to you, there's often something there in their eyes when they look at you, sort of a, "yep, you're one too!" look. Hard to explain. But that's why this works for me even if the person in question is not attracted to me or if it's a gay man rather than a woman.
DB, I like your "Jenndar" idea!!!
I feel the same way about being Jewish. I can always tell when someone else is, or is not. It's so easy.