There are lots of reasons why you might not want to just come right out and ask someone whether they're gay. Maybe you're super curious about a coworker or acquaintance you're crushing on, but don't want to risk making them uncomfortable. Maybe you're just not the type to ask people really personal questions about their love lives, but that hardly means you don't want to know.
Thankfully there are a lot of little tells and habits most non-open gay or bi guys have in common, so understanding what to look for can be pretty helpful in making an educated guess. The following are some good examples to be aware of.
1. He doesn't say much about his social life.
Sometimes it's not what someone says, so much as it's what they don't say. Every gay, bi, or queer person is different when it comes to how "out" they are. Some people are so open about being gay, that it's impossible not to know, but others are a lot more private for any number of reasons. Many closeted gay men keep very quiet about their social life, especially regarding who they live with or date. Of course, there's always the chance they're just private, but even private people usually talk about people they're close to at least some of the time or mention them in passing.
2. He speaks a little too fondly of a specific friend.
Most people have known someone with a same-sex roommate or good friend they seemed unusually close to. They talk about them a lot and in a way that makes it hard to believe there's not a lot more to their relationship underneath the surface. (You may even have been that person yourself and know from experience.) If you really do get the impression someone's a lot more than friendly with a same-sex bestie, there's a good chance that you're right, especially if other friends and acquaintances share your suspicions.
3. You could swear he's flirting with you at times.
If you're not openly gay yourself, another gay man who also keeps relatively mum about his sexuality may not be sure of yours either. However, he might well try to drop little hints now and then to see if you pick up on any of them or divulge any information on your own. Does he sometimes touch you or get super close to you in a way that doesn't feel accidental? Does he ask you probing questions about your sex life? If know he thinks you're straight, do you get the feeling he's trying to see whether or not you're into "experimenting" with other guys? There's a good chance he's looking to get a better read on you without actually outing himself in the process.
4. He doesn't mention being attracted to women.
Again, some people are just really super private about their personal lives, especially when it comes to sex or romance. However, most straight men will at least occasionally talk about their attraction to women. They'll talk about what they like or comment on someone specific they find attractive, especially if they're in the company of other men talking about those things. If you've literally never heard this guy say anything along those lines, there's definitely a chance that it's because he's not actually attracted to women.
5. He seems awkward around gay men.
If you're gay and the guy you're curious about knows it, he may sometimes seem awkward or shy around you in a way he isn't around men he knows are straight. He avoids making eye contact and may seem self-conscious in general in a way that doesn't make sense. Alternatively, he may seem this way around other gay men you know. Closeted gay men and gay-curious men who are still in denial about their sexuality often worry that other men like them will instinctively be able to recognize one of their own. (Sometimes that's the case, but other times it's a bit more difficult to tell.)
6. He seems obsessed with gayness despite being straight on paper.
You're no doubt familiar with the stereotype of the homophobe who's secretly gay himself. Sadly, there are still a lot of self-loathing gay men who attempt to hide their sexuality by making sure everyone knows just how much they hate gay people and disapprove of the "gay lifestyle". And they sure do talk about gay people a lot for people who are so darned straight. If you know someone who's like this, you're right to wonder whether they're gay, because they probably are.
At the end of the day, no two gay people are exactly alike, so the only way to be absolutely certain is for the person to tell you. However, there are often enough common points that it's possible to make a very good guess. How does your friend measure up?