A certain tone of voice can sometimes be a tip; also some effeminate hand actions, etc. I do agree that many gay men do not appear to be gay by these criteria. OTOH, there are some gay men that seem to be obvious to me.
A man stranger who smiles at another man for no reason.
Touching another guy during conversation. Especially his hand or knee is a dead give away.
Telling another guy he is strong, handsome, smells good or he is really funny.
He listens to gay sounding music, has gay hobbies or he is a gosip.
He has been to in New Orleans, Austin, Key West and San Fran.