I scared a gay guy away once by slipping him my number.
I mean, we were on the bus once and he'd look behind his self just to look at me. He really looked at me, he was checking me out. I could see him looking me up and down and he didn't care that I caught him doing it because he kept at it. This went on for about a week before he decided to sit next to me. It felt so awkward, especially since he didn't sit all the way back in his seat. I could see him glancing at me from the corner of his eyes. When the bus got closer to my school, I put my number on a piece of paper and dropped it behind him in his seat since he wasn't sitting all the way back.
I'm not sure if he called or not because I received a couple calls from unknown numbers that day. When I saw him on the bus again he didn't look at me once and hasn't sat anywhere near me since. Even tho it didn't happen, I'm glad I didn't let the chance slide by as I usually do. I haven't let any opportunities slide by since then.
Anyway, I'd say prolonged eye contact. It's not foolproof, but it's something to go on.