Recognize anyone??
Notice if your plans are continually overturned in favor of his/Her's. Instead, you're always changing plans to do what he/she wants, always meeting up with his/her friends.
Be aware of the way he/she behaves with your family and friends, especially if he/she interrupts them, contradicts them, or behaves dismissively. If you feel you need to apologize or explain for his/her's behavior to your family or friends, there's a problem there.
Are you realizing it's just become easier not to spend time with people you've loved for years, rather than to make apologies or excuses. #* Have all of your past attachments to people and places been replaced by either old friends of your new love, or new friends you've made since you've been together? Severing your ties to the familiar stability of people you have always known means he has just made himself the center of your universe, and now has no competition for your attention.
Watch out for subtle discrepancies. When talking with mutual friends, have they ever said something about your new girlfriend that made you stop and say, "Huh? But he said something different to me... You can't have understood that right." Did you then dismiss the idea that what your friends heard could have actually been true? That's a big red flag.
When you're being controlled or manipulated, it's usually through half-truths or omissions, not outright lies. There's just enough weirdness to make you stop and think, but not quite enough to get you to re-evaluate the entire relationship. If this happens more than once, STOP and remind yourself that this isn't the first time you've had this reaction. Start analyzing discrepancies between what he said, and what your friends say.
If there are a lot of them, call her out on them. If her reaction or answers don't satisfy, it is time to re-evaluate in a major way. And don't waste time doing the analysis - it may save you from disaster later.
Cutting you off from your support systems helps him gain dominance over you - and you think it's your decision. A controlling partner will treat your friends with disrespect - your friends will report rude remarks made behind your back, or you will actually see him treat them in a dismissive or outright rude way. However, when you're alone with him, he never says a bad word about those friends, but rather is kind, loving, and complimentary to you about them.
It makes you believe your family or friends are simply jealous, don't understand him, etc. You forget his nastiness to their faces because he's nice behind their backs. When you find yourself telling your mom or sister, "But, you have to understand him like I do," that's a bad sign. Why should everyone else understand him and adjust their behavior - wouldn't it be easier if he would adjust his? It's much easier to for him control you when you've decided your loved ones just don't understand your mate, and soon, you have no one but him to turn to.