When you have a gut feeling, when they act different on you, when they give in when you interrogate them, when they start distint themselves from you, when you stalk them and see them making out with another guy, when they don't want to do the things you guys always would do before, when she start acting funny around her friends etc...

Lying is usually the BIG sign of cheaters.
Example:i have work, and they aren't at work.

You find her talking a lot to a certain person, talking about the same things doing the same things, when you guys first hooked up.

Or just by saying listen, I feel like you might be cheating on me and we need to be open here. And the way she answers back, the pauses the look on her face etc will give it away.

If you have to ask total strangers this question, you don't trust her. You can't have a relationship this way. You need to talk to her, let her know what little clues or supposed signs are bothering you, let her answer, and take it from there - you either believe her or you don't. If you don't, there's no point in arguing, you just leave. Keep in mind that some mistrust is well founded and other mistrust is based merely on one person's long standing inability to trust. Beware of letting past hurts and family history cloud your judgment, but don't be a fool for love either. (Easy for me to say, I know, it's a lifelong job for most of us).

Why do you think that she is cheating?