Unfortunately, cats can't tell us when they're in pain--they would if they could--because they hurt just like we do--therefore, it's up to us humans to figure it out. Here are some signs to help determine if your cat is feeling pain.
Instructions
A lack of appetite is usually the clearest indication that a cat is in pain or has some type of medical problem. If your cat is not eating or seems to have trouble eating, make a vet appointment ASAP.
If your cat is quiet and unresponsive or just not his usual energetic self, this could be a good sign that your cat is in pain. A couple of days of this behavior also warrants a visit to the vet.
If your cat is hiding in an unusual place, this could be an indication that your cat is in pain.
If your cat is avoiding you, other people and pets, there's a good chance that your cat is in pain.