There are many different ways but the easiest way to know is that they will try and make you feel the worst and will tell others to do the same thing . also they will try to focuse all attention on themselves not on you .
If you are with someone else then it really should not matter what your ex thinks either way. If you are trying to make him jealous and just using someone to do so that is wrong and not fair to that person. If your ex is jealous you will know by the way he treats you.
Tell that jealous person that if they really liked you then they would only want you to be happy, and that your girlfriend makes you happy.
If the person begins to seem a bit angry at you and doesn't tell you why then it's probably because they are becoming jealous at what you are doing.
Ask them
try and cause trouble with the guy your dating
tell him there's nothing to be jealous of.
when they call you. when they ask to see you. when they look into your eyes, and you get that feeling. when they ask your friends about you. when they get jealous seeing you with someone else. the list goes on.
either he want you to be jealous, or he thinks of you as a friend and wants to tell you things that he can trust you with.
if its a girl...tell her that she doesnt deserve to look at his beautiful eyes...if its a boy tell the truth
People are jealous because someone else is getting something or even someone that they cannot get.
You can tell him that you like him or find another guy or you could try to make him jealous.
Make him/ her jealous.