How can you tell if a guy is jealous? What are the signs?

I'm trying to figure out a guy I know. He is only an acquaintance and he doesn't act like he is interested in me. He's nice to me but is flirty with the other girls. Once when he and another girl and I were talking, he turned to look at me and just stared at my eyes for longer than it was... Show More

I am pretty sure he is interested in another girl we know. So it doesn't make sense to me that he would be jealous about me...if he is.