Trust is one of the most important parts of any relationship. If you have reason to believe that your girlfriend is violating that trust with promiscuous behaviour, you need to know for sure. Before you make accusations that could ruin a relationship, look for some of the telltale signs of promiscuity to be certain you aren't just being paranoid. You can tell if your jealousy is unfounded or not by taking a few easy steps to find out how your girlfriend acts when you aren't around.
Look to her friends. If your girlfriend's friends engage in promiscuous behaviour and she spends a lot of time hanging out with them, odds are good she enjoys the same behaviour. Do her friends hang out at clubs and pick up guys? Do they talk a lot about different men? While it doesn't guarantee your girlfriend is promiscuous, it's a good indication.
Watch for signs that she is keeping something from you. If she refuses to tell you where she was on any given night, it's a sign she was with someone else. While everyone is entitled to privacy, regularly keeping her whereabouts a secret is a red flag.
Check her out when she comes home if she lives with you. If her make-up is a mess and her clothing looks rumpled, it could be that she was just dancing all night, but it could mean she was with someone else. Strange smells on her clothes are another good indicator.
Ask her ex-boyfriends. If you happen to know people she dated before you, they are the ones who can give you inside information. If she cheated on previous boyfriends, odds are good she will cheat again.
Ask friends to watch her. She probably won't behave in a promiscuous fashion if she knows she is being watched, so ask a friend she doesn't know to follow her when she goes out, and report back on her behaviour.
Be subtle when checking up on your girlfriend's behaviour. If she isn't doing anything wrong and finds out you have been looking into her activities, odds are she will be angry.
Never do anything illegal in pursuit of information about your girlfriend's activities, including opening her mail.