how to ask someone if they're gay secretly

make a gay joke in front of them, this will involve revealing yourself as gay but if you're comfy with that then it should work

we need some kind of signal to tell other gay people that you're gay dkjfhsjfhsd

i asked my friend secretly and turns out hes gay 

Just ask it, I don't think is a offensive question to make. You have to do it in the right context, but that applies to any other question, make clear what are your intentions asking that (curiosity, romantic interest, sexual interest...). 

For the lesbians/bisexuels/wlw:

“Do you listen to girl in red?” 

Do you like bsd? Or do you like Gigi the artist of sailor?