If your girlfriend is wanting sex out of no where all of a sudden or just the oppisite if she's suddenly not interested when normally she is, then yes you have a problem. Also, if she has pulled away emotionaly and seems distant and there is no time for conversation, kisses, not saying I love you as much and not coming to you for advice or help then it is very likely she's cheating. Most importantly if she has become unavailable to you as far as not being home hardly, spending more time with friends and not answering her cell phone then yes it is possible she is cheating. If there are kids involved then you might be able to tell if she is cheating, if she's neglecting them or much much more involved with them then before. Hope that all this really helps you out. If you just want to cut to the chase and put it out there, you could simply just ask her and if her reaction is, she pauses and kind of swallows hard then doesn't make any eye contact with you when she answers then it should be written all over her face and if she gets very defensive then she is definetly cheating. Lastly, something that would be the best answer to your question is ask her to merry you and if she is kind of cold and non responsive, hesitates or comes up with an excuse then yes she is cheating. That is if you want to take a chance she'd be exstatic and jump into your arms in tears saying yes and then you will know you have done all this to find out she is not even cheating on you. Well GOOD LUCK!