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... Show More

Updates:
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.