If you suspect someone to be gay, chances are, other people will too. A human knows what they are doing and why. Another thing that I look for is homophobia. If he doesn't, then calmly ask if he might be gay, and say that you only want to know because you want to support him and you want him to be open with you. If you're wondering if your straight friend's significant other is gay, then you should leave it alone, too. They're still the same person! If you ask him out, that isn't the same as assuming he is gay.
If all else fails, sit him down in a nice, relaxed room.
Don't scrutinize the way the person talks. Connect with me on LinkedIn. You're not exactly sure if he's into you, but fingers crossed he finally makes a move during this digitally-remastered afternoon showing of "The First Wives Club. If you're childless get away.
Michael So True!