Everything Turns into a Fight
If it seems like the two of you can't even have a conversation without it getting ugly, something is wrong. This does tend to happen with every couple on occasion when one or both are under a great deal of stress, but if it goes on like this for months, all you're doing is making each other miserable. It's likely that both of you have some pent up emotions you're keeping inside, and not letting it out is causing you or your partner to blow up over every little thing. The two of you need to have a heart-to-heart if this is happening, and say what you're feeling. You'll be surprised at how much better you'll feel when you just let it all out.
Your Partner is Making Excuses Not to See You
Everyone occasionally has things to do other than hang out with their partner, and that's OK -- sometimes. If he never seems to have time for you anymore or always has something else he has to do rather than be with you, you should ask what's up. Unfortunately, it could be that he is intentionally trying to avoid you. If he would always rather hang out with his friends over you, then you have to decide if that's OK with you. The next time he's busy, ask him what he's doing and if you can come along. If he says no, ask him why. He probably won't have a good answer if the truth is that he just doesn't want to hang out with you. Keep in mind that this is OK sometimes, everyone needs a "break." Don't freak out if it happens once a week. It should only be a red flag if it happens so often that you don't see him for a week or more.
Loss of Affection
When your partner no longer kisses you, touches you, hugs you, or tries to be affectionate with you in any way, something is definitely wrong. This is especially true if the other person was very affectionate before. For whatever reason, they don't want to be too close to you, and you need to find out why. Don't waste time - call her out on it. Ask what's going on. You may or may not get a straight answer, but the important thing is that you brought the issue to the table. It's out there, and can no longer be ignored.
Something Just Feels Really Wrong
Instincts are often right. If something just feels wrong or off to you, then it probably is. If you know your partner well enough that you can sense when something is going on under the surface, call him out on it. If what is going on is that he wants the relationship to be over, he'll probably say so if you ask him. It won't be an easy conversation to have, but if he wants out it's going to happen sooner or later whether you like it or not.