You can a guy is jealous by his body language. If he acts as if he does not want to talk to you after spotting you with another guy, he's definitely jealous. If he also asks you many questions about the guy then it's clear he's jealous.
If he is jealous.. he wouldnt want you to hang out with your friends,he would want you to be with only him all the time, he would want to see who called/text you (he'll take your phone), doesnt want you around guys, gets mad when you have guy friends, if he see's you with your guy friend he wont want to be around you anymore, he walks away when your around certain people .. just like reginamuthoni0 said! i know i had this happen to me once