Be wary of any relationships she has to which you may not be privy. For example, you might not be friends with all of her friends, but you certainly know who they are. If your girlfriend talks about a male friend whom you've never met -- and talks about him often -- then you could have a problem, particularly if she is resistant to the idea of you meeting him.

Pay attention to her body movements and facial expressions when she says she loves you. You will know if she means it; if she looks pained when she says the words or can't look you in the face or even say it back, when she has been able to do all of that in the past, then something is amiss. She might love someone else if those same loving facial expressions that were reserved for you are now apparent when she talks with another person.

Observe her behavior. Pay attention to whether she has become more considerate or attentive to someone else and less so with you. For example, if she is not willing to do activities with you but can give that time to someone else willingly, then her feelings for you could have changed dramatically. If she was once kind and gentle with you but is now brusque and impatient all the time, but around others, she is not, then your relationship is likely on the rocks.

Confront her. This is truly the only way to really know and, while she might verbally communicate one thing, the truth will be on her face. If you ask her if she is in love with someone else and she denies it, but gets flustered or cannot look you in the eye, then you have your answer. If she is overly defensive or dramatic, you have cause to be suspicious then as well. When you confront her, just ask. Don't be accusatory or mean-spirited; simply explain why you are asking and encourage her to be honest.