They will act different the regularly
Most likely, they will tease the hell out of whatever they are jealous of you for.
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.
show him what hes missing make him jealous ;)
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.
Not sure how you expect someone to magically know something.
People are jealous because someone else is getting something or even someone that they cannot get.
Make him/ her jealous.
A jealous significant other is someone like a girlfriend boyfriend, husband, wife that is jealous of you, if you get what im saying :s basically someone significant that's jealous bout something.
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.
if someone pretends to be jealous it is usually really false and over the top :/ i have alot of friends that do that and they all take it too far with what they are doing...look out for them being over dramatic
You may feel jealous of someone if you have negative emotions such as resentment, insecurity, or envy when you see their success or possessions. Jealousy can also manifest as a desire to have what someone else has or feeling competitive with them.
Jelousy is a common feeling and you don't have to love someone in order to get jealous. You could hate someone and be jealous.