When you look at your phone and you have 20 missed calls

when you have voice mails with nothing but "where are you"

when they show up at your job constantly

when they can do the things you cant

when you're not at liberty to share time with others as much as the other person does

when you cant go anywhere without them

all these are sure signs of a up and coming break up!

If a guy starts telling you what to do, when to do it, how to do it, what you can and can't wear, who you can and can't talk to, how to do your makeup and hair... if he gets mad just because you hugged or talked to another guy, if he constantly checks your phone and computer behind your back (considering you give him no reason not to trust you), if he seems to get mad at you for the littlest things, if you have to start giving up going out with your friends for him... it's things like this, when he's pretty much trying to tell you how to live your life, that a relationship is getting controlling.

Of course, there are some exceptions.
He shouldn't care if you have guy friends, but it's not controlling if he doesn't like you hanging out with another guy, one on one, alone.
He shouldn't try to tell you what to wear, but if you're walking around in shorts that barely cover your butt and a shirt that shows about 3/4 of your boobs, then he has a reason to be upset.
He shouldn't be asking you to give up going out with your friends for him, but if you blow off the plans the two of you already had for your friends, or if he invites you to something super important to him, but you choose to go to the mall with your friends instead, he has the right to be upset.

Just things like that - remember what's acceptable and what's not, and don't let him control you.