He's mean. He's sweet. He's quiet. He's loud. He's around all the time. He's absent. He acts happy. He acts sad.
Sorry...this isn't helping any, is it?
(I guess it depends on the guy. I think that they express jealousy in different ways, and it depends on his personality and the way he deals with strong feelings or people in general. Some are confrontational. Some passive. Some will compete. Some will give up and withdraw. Some will ignore you, and some will get up under your feet all the time. You have to know something about the guy.)
I've known guys who've done all of those things. Sometimes the same guy will act in two totally different ways because he hates being jealous. One thing you can do...ask one of his other male friends. They can usually tell these things better than we can.
When he says he's not