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 comfortable, with this blank look like his mind had suddenly gone blank... Sometimes I've heard him bring me up in conversation with the other girls (nothing bad, just like he was trying to make sure I was included), and I was in another room...why is he even thinking about me since we aren't really friends?

All the girls flirt with him, except me. (I'm not confident so I don't flirt with a guy unless I am sure he likes me at least as a friend.) But he had to know who a guy was that was being a little flirty towards me, and seems slightly upset when I am really friendly with another guy we know. I'm guessing he might be so used to having all the girls flirt with him and it upsets him that I don't, and even more that I do flirt with other guys?

How do you know if someone is jealous? What are the signs?

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.