Cats can’t tell us where or when it hurts. In fact, many animals will try to mask pain or illness. That’s why it is extremely important that you get to know your cat’s regular habits and behaviors. Any deviation from his or her normal behavior could indicate a problem with his or her health. Some symptoms and behaviors are obvious: diarrhea, vomiting, coughing, sneezing, runny nose or eyes, and trouble using the litter box. There are other, more subtle, physical and behavioral signs that might indicate a trip to the vet is in order. Take note if your cat does any of the following:

Misses more than one meal
Shows a sudden change in eating habits
Stops using the litter box
Develops puffiness or a lump under the skin
Hides for more than a day
Becomes suddenly short-tempered or intolerant of attention
Shakes his head frequently
Changes his routine or loses interest in his favorite games
Stops grooming

All information from http://www.hsus.org

One thing that I would add would be an increase in the amount of water drinking and subsequent urination.  Just as in humans, this can be a sign of diabetes.  As the “staff” for now my second diabetic cat we caught this one much earlier in her disease process because I noticed that I was seeing much more and much larger litter clumps.  Skin problems and frequent scratching can also be associated with diabetes.  Our current diabetic cat is now 16, gets 2 Vetsulin (insulin for animals) shots a day which she doesn’t even seem to feel, and, other than having about the same degree of arthritis as I have, is doing just fine.