If your worried your girlfriend might be cheating then there is pretty good odds that she is because it usually takes a lot to convince yourself someone would go behind your back with another guy.
I would assume there has been some signs that make you wonder or perhaps they have done wrong in the past and are acting differently.
These can all be signs of infidelity, but without real evidence it's best to not say anything because you could be wrong and ruin the relationship or they will cover there tracks when they know your onto them.
Different women will leave different signs, but most women are fairly attached to there boyfriends and want to spend a lot of time with them.
If this isn't the case you have to ask yourself if this is normal because of there schedule or if there spending it with someone else.
Another thing you should keep an eye out for is how affectionate your girlfriend is being because if there is no spark there she might not be happy and seeking attention from someone else.
Keep an eye out for where she says shes going to be and if she comes home with receipts from somewhere else or smelling like mens cologne you know there is pretty good odds she isn't being honest with you.
Things like smell are overlooked by clumsy cheaters which helps guys who are smart and well prepared catch them in the act.
If your girlfriend has a cellphone and you think something is going on take a look for any unusual numbers or text messages, but make sure there is other signs first because invading your girlfriends privacy isn't usually a good idea if there honest.
It's usually the simple things and your gut feeling that lead you to believe your girlfriend is cheating and these are the best signs of all so do your homework and don't do anything too crazy until you have some evidence.