I've had a man shake with rage and tell me how much the other guy bothered him (by just talking to me and not even flirting as I was sitting on my boyfriend's lap at the time).
I've had a man tell me he's uncomfortable with me being around another or other particular men without him being in the group too.
I've had a man get really quiet and physically tense up (jaw, biceps, etc) just at the mention of another guy's name (just a friend). Then I would say something to reassure him, and it was like it never happened.
I've had a man tell me that he hated me drinking around other men or dancing with other men.
Different men are comfortable or uncomfortable with different things. Some guys didn't care what I did so long as I didn't sleep with anyone. Some guys didn't want other men to look at me in a way they thoughts was inappropriate. (I've never cheated on anyone unless you count briefly dancing with someone else.) They also all express jealousy differently.
I don't know dont really know most of the time i keep 2 my self if im jealous shows weakness towards the girl. And weakness is a major turn off for girls and put you from them liking you in a dating way 2 your a friend for ever
Depends on the guys personality really. Some men will look at a woman they like with another man and then 2alk away and avoid that person completely which can be confusing. Other men may react angrily and confront the woman or nan or both if he feels threatened by their interactions. It's safe to say that if a guy starts acting differently around you compared to before in a romantic sense and it involves competition then they could very well be jealous. Always best to ask outright though rather than make assumptions.