How can you tell?

[nullnaught]

You have to be very perceptive. One way to tell is to observe their actions/reactions when a member of one sex walks by compared to the other gender. If they are 'in the closet' or 'straight acting' you will have to watch closely. A tightening around the eyes, a slight widening and how long their eyes stop and focus on the person walking by.

Where they look (at the person) is also telling. Few straight guys will let their gaze fall to scan the body of other men, gay/bi men are more prone to do that. Its 'checking' the person out. This could be nothing, there are some people who automatically scan a person up and down (a once over) to assess the person by their fitness, clothing, posture. Someone who has an interest in that gender will do a once over, followed by either furtive glances to all out deep scanning.

Conversation can be most revealing. How they lie or don't about their partnerships. You have to focus on finding the tells of lies when questions are asked.

Pupil dilatation is also telling, of course you need to be up pretty close to see the changes, but a lot of data can be gleaned from the tiny movements of the pupils.

O.K., I must admit I have a huge disadvantage here, because I have a genetic inability to "read" people. It is called autism. So my answer above was biased. But I know people who could supposedly read people like this who got it wrong, and nobody was ever able to tell I was gay before I told them. Most people have laughed and said I was kidding. I am just saying that looking is never for sure. The only way to know for sure is probably to have sex with them.