How can you tell if a man or a woman is gay?

Answer 1

Asking him or her is the beet way to know for certain. But realize that asking a person such a question may result in a response like "it's none of your business." To say that someone is gay based on anything else other than what he or she says, is to propogate a stereotype.

Answer 2

You ask them. Anything else is just an assumption.

However, how is this in any way your business, unless you plan to ask them for sex?
Or to redecorate your apartment, or tune your engine? [Oops, sorry, stereotypes, bad! bad! :o) ]