If his name is Parsifals =D
Look at his poster collection.
Does his **** taste like s88t
Do you want him to be BI, or are you afraid he is cheating on you with another man? There is no surefire way to know unless you ask him. if you cannot accept it than you may want to let it go and let him go.
If he comes home smelling of, welllll, you know.....
He says "I'm bi."
Seriously, no action, no stereotype will make him fit a certain category.
if you walk in and he and another dude are nekkid thats a pretty good indication.... thats how I found out ... *shudder*