How can you tell if a guy's gay or not?

What could be telltale signs that a guy is gay, given that he still hasn't come out of his closet?

I know this guy in high school who was quite the ladies' man. He is fair-skinned, good looking, very charming, and smart. He was easily one of the campus crushes then. The only thing effeminate about him was his tone of voice, which was kind of high-pitched and whiny. Since he was an all-around Adonis everyone overlooked this trait on him, even the guys.

However, I heard just recently a really shocking news about him. He was out partying in this trendy club usually frequented by gays. I said to my friend who told me about it (who is proudly gay) that he might just be mistaken, that maybe Mr Campus Crush was just there because he lost a bet or somthing. But no, he was dead sure it was our little friend from high school, as 'Adonis' cheerily waved to my friend in greeting while gyrating his entire bod on his partner (who we soon found out was his boyfriend!)

We weren't close, him and I, in high school even if we were classmates but even close friends of his didn't expect him to turn out that way. I don't know if he was already gay in high school then and was just pretending to be macho and all that, or maybe his turning point was college. We just didn't see it coming.

Do you know guys whom you thought was 100% straight but then turned out to be totally gay? How can you tell if a guy's gay or not?