Here are some phrases that should set off huge warning bells in your head if your guy ever says them:
"That skirt is way too short. Don't go out looking like that." Only your parents are allowed to tell you this one! But your boyfriend should never worry about how you dress - that's your business and your business only.
"Why are you still talking to your ex-boyfriend?" Jealousy!! If you notice that your guy is getting upset when your ex calls you, it's a bad sign. Make it clear that you talk to whoever you want to.
"I don't really like your friends, so I'm not going to hang out with them. Let's go out with my friends instead." This shows that he's super posessive and he might even want to distance you from your friends. Let him know that your friends are really important for you, and you'll keep hanging out with them whether he likes it or not.
"You could go on a diet to lose weight, couldn't you?" Guys should never pressure you into changing your body for them. A guy, if he really loves you, will never tell you this kind of stuff. And if one does, tell him to get lost - he doesn't deserve you.