Worn teeth are usually darker in color, oddly shaped, or worn down to the gum line. The teeth most commonly affected are the incisors and canine teeth. The incisors are the small teeth in the front of the mouth, and the incisors are the "fang" teeth.
How are teeth worn down?
Teeth wear down by rubbing on each other (a malocclusion or "bad bite") or by the pet chewing on their fur and skin (as in the case of itchy allergies) or by the pet chewing on items (pet toys, bones, sticks, rocks, etc.).
How quickly does this occur?
This change usually occurs gradually, and the tooth responds by laying down additional dentin to harden the injured area. In these cases, the teeth are normally left alone unless they are painful or the gum is infected.
In the case of sudden wear or a fracture of the tooth caused by chewing, your pet should be examined by your veterinarian immediately. Depending on the nature and extent of the tooth injury, your vet may elect to extract the tooth or preform an endodontic treatment, also known as a root canal, just like in humans.
Getting to the root of the problem
If the teeth are very worn, your veterinarian may want to do dental radiographs even if your pet is not showing signs of infection or discomfort. Radiographs will rule out a "dead" tooth that appears fine on the outside, but not vital on the inside. This could could cause problems later on.
Additional information and excellent photos of worn teeth in a dog:
American Veterinary Dental Society - Worn Teeth