There aren't any foolproof methods of determining if a man is or will be gay (same with women being lesbians).
There are some assumptions... man dresses like a metrosexual or never has a girlfriend... woman dresses like a tomboy or never has a boyfriend.
Sometimes, it could be true... or not. It's like the famous quote... "Don't judge a book by its cover, but what's in it."
I've met guys who never had girlfriends, but that was during their college years where they were under heavy pressure by their parents to focus on school or felt that they had to do everything they wanted to do without dealing with women. They ended up graduating and getting a girlfriend years later... and getting married and having kids. I've met guys who dated women and were all macho who drink beer and wrestle all the time. Yet, they ended up being gay years later. Maybe, there was a life-changing experience that led that person to become gay or maybe he was just in the closet for years and decided to wait until he graduated before coming out of the closet.
There are some fraternities who are very anti-gay and some members don't come out of the closet until after they graduate out of fear that they will be targeted by their fellow brothers of the active chapter. Some women will see that fraternity and say, "Oh, that fraternity never has gay brothers because they are all real men that women want." Well, they don't know the whole story... that's sad.
