I'm not really sure about this, because I've never had very good "gay-dar", but you can't live your life in a bubble simply because you're afraid a guy MIGHT be gay. Most of the people whose marriages fall apart due to one spouse being homosexual, actually say they had a wonderful marriage and would never take it back. Some even continue to be friends with their ex. It's a chance everyone takes when we date/marry, but it's worth the risk. Chances are, you'll marry a guy who's as straight as they come. Just don't worry about things like this. Have fun dating, and don't stress over who's gay and who's straight.
After all, some of those people who end up being gay after being married for years never exhibited any signs of being homosexual. They were always the guy's guy or a girlie girl, fawning all over the opposite sex, but it all turned out to be an act. So, just because someone acts stereotypically gay or straight doesn't mean they are. Take a chance and date who you like. If you watch too long, you'll miss out on the opportunity to make your move.