How do you know if your girl friend is cheating?

I do not know the situation you have with your girlfriend, but for you to ask what to look if she is cheating, why don't you just ask her. Make time to see her and just talk. Communication is a must when it comes to relationship. Ask her if she feels neglected by you, or maybe she feels that you don't have time for her anymore. Finish what you needs to be said and only this way you can find out what's going on in her head.