How to test if my friend is gay?

i'm a straight girl, and me and my friend are like 90% positive that our guy friend is gay but we aren't sure. everyone thinks he's gay but he hasn't come out yet. he's not like a super lisp fashionable gay (he wears ed hardy though and pink shorts) he has a deep voice, most of his friends are girls, he is very funny and guys think he's funny too. however are there any tests (not like hooked up to a machine test) that i can subtly do to help prove he's gay? and NOT so i can laugh at him or whatever or go spread it around school, just so i can have the urance. and i'm not going to be seductive and see if he's turned on. nor will i have a boy try to seduce him.