You cant. Some gay men are so masculine (like most of you guys here) there no way you can tell. Of course if he holding some guys hand , maybe there a posibility. You can only be sure if he admitted it to you. Who will thought hunky cowboy Adam Sutton (google that) is gay,
Zeb Atlas, Rock Hudson, Rupert Everet, Bob Paris, Esera is gay. Gay man is everywhere , and they are wonderful.