How cool is she?
Some women — especially the jealous types — are relentlessly insecure and interpret any male independence as a threat to their sexual control of the relationship. This includes spending time with your buddies whom she views as a gateway to the single life and opportunities for interaction with other women. Guys like to go to sports bars, strip clubs, games — places where she can't exert her control. Test her by telling her you can't see her Saturday night because you and your friends are going to a strip club. Her reaction will be immediate — but again, prepare for he worst.
Leave her alone with your friends or family members to see how she interacts with them. Is she polite and engaging? Does she make an effort to get to know them — or does she view them as competition? Ask them what kind of impression they got from her, and pay close attention to their answers — they may be seeing something you're not.