How can you tell if a guy is jealous?

Anonymous
I like this guy and Im wondering if eh likes me back
I sometimes tell him about my (male) friends and whenever I do that he starts to act strange...
he makes fun of them (even if he doesn't know them), he tries to make me step away from them and he often changes the subject of the conversation

Does it mean he is jealous of them?
What makes a guy jealous
What makes a guy jealous
How can you tell if a guy is jealous?
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