A straight guy knows when another guy looks good and checks him out, even if he's not gay. Face, clothes, body, cock presence or what have you, all look and check things out. The ones that look excessively are the ones that make you wonder, me rather. I have a boss that looks me several times up and down every time I talk to him and it's quite noticeable. I believe he's at least bi, could be wrong.
Then there's the guys you see looking at you and very obviously jerk away from looking when you look at them. Those always make we wonder. Then there are those who seem they would never establish eye contact for fear you may think something if you see them looking.
All interesting.