A healthy, well-groomed cat has a soft, clean, slightly lustrous coat. A cat whose fur is dull, dry, oily, or unkempt may just need you to step up his grooming routine, or he may be under the weather. Even with regular grooming by humans, a cat needs to do some of his own grooming to keep his coat looking good. Cats are usually pretty diligent about their personal hygiene, so a cat who's not keeping up his appearance may not be feeling well.
On the other hand, a cat who's grooming himself raw is also telling you something. Excessive grooming can be a sign of stress, a skin problem, or a reaction to fleas. Look for "hot spots" -- patches where your cat licks so much that the fur is gone and the skin is red or raw. This condition, called moist acute dermatitis, can be a sign of many different kinds of conditions. These may include an insect bite, a flea infestation or an allergy.
Of course, not all hair loss is from grooming. Take note of any bald patches or areas where the hair is thin or sparse. Most of your cat's body should be covered with a coat of hair thick enough to hide the skin underneath. (About the only place where it's normal for the fur to be thinned out is the area between your cat's eyes and ears.) Whatever the case, your vet's advice will help put your cat back on the path to a healthy coat.
While you're checking out your cat's coat, you should also do a quick assessment of his weight. Run your hands over the cat's sides -- in a cat at a healthy weight, you should just be able to feel ribs. If the ribs are very prominent, your cat could be underweight. If you're having trouble finding ribs, then he or she could be overweight. Just as in people, an overweight or obese cat can develop a host of health problems. Discuss any concerns about your cat's weight with your vet.
Do you brush your cat's teeth? If not, the next section might make you think about giving it a try.