I have an obvious example of when my close friend was jealous of me and she TRIED to hide it but was unsuccessful by how grumpy she was over the whole thing. Also, she would always bring the topic up behind my back to family members and friends in some form of gossip. That's when you know they are jealous of you because you are living in their head rent free and they find it to be a topic of interest to talk about you, gossip about you, or complain about you, with the main conversational point revolving around YOU instead of talking about herself, her own life, her own goals, relationships, etc.
They will also try to hide their gossip/complaints as "concern" so they don't look like the bad guy.
For example, I am very happy with my boyfriend but when I first introduced him to my family and friends, she would be fake nice to our faces and say "I AM SOOOO happy for you!!!" "WHAT A CUTE COUPLE" and other shallow compliments but then complain about us behind our backs, saying we are "too co-dependent", "he's too controlling", "she can do better", "I don't like the look of him" or other superficial lies that are masked as concerns. (Jsyk not in an abusive relationship or anything but with the way she talks about us it SOUNDS like it. When she complains about us to her friends and then they meet us, they look at her like she's crazy because we are by all means a normal functioning couple).
Even in casual friend gatherings when I'm absent, I am told that she brings us up all the time. I don't know why she talks about us a lot unprompted. My only guess is that we are on her mind because she is jealous that I am not single anymore and have a successful relationship when she doesn't? I think she seethes because my relationship has progressed far faster than hers has with us moving in before she moved in with her boyfriend (and that's literally her obsessive goal right now).
I think there are types of people in the world that use the people around them as measurements of their own self-esteem, success and confidence. For example, I was single and a turbovirgin. Being the loser virgin friend allowed her to feel superior and accomplished. When that title was stripped away by me entering a relationship (and not a crappy one either), she suddenly feels inferior. Her friend, a loser virgin, is suddenly not a virgin anymore and has progressed and reached a relationship milestone before she did. It made her feel crappy, unaccomplished and look bad in compaPost too long. Click here to view the full text.