Are there signals for gays to know when another person might be gay?

Yes. It's called gaydar and you either have it or you haven't, or you pick it up with experience. Difficult to describe, it's a mixture of instinct, eye contact, body language, chemistry and visual signs that are so subtle most people would not see them. It is not fool proof and should only be acted upon if there are other indications.

If you are in the company of the guy and you want to know if he fancies you, prolonged eye contact is the usual way of attracting a guys attention, but again, be sure it's a sexual situation and not a potential threatening one. Stare at a straight or nutty guy and fists will probably fly. Once you get your potential guy close up, if his iris in the centre of each eye is enlarged (from the usual tiny black dot) it means he likes what he sees. Then it's up to you to make your own not so subtle signals!

Only two surefire signals I know of:

1) The other person has a same-sex partner with him/her.

2) The other person admits to being gay.

Well if they are with their boyfriend or girlfriend that's usually a big signal...

but beyond that there really are not any other than flirting and the like.

rainbow belt buckle, skinny jeans

In less they tell you , no . Or not from the naked eye anyways :)

no