One side should be the same as the other. Note abnormal swellings. When looking at the head, feel the glands under your pet's neck. Both should be the same size. Next, raise the lips and look at the teeth and gums.
Cats are especially prone to painful cavities which occur at the gumline.
Finally, if you feel courageous, take a whiff of your pet's breath. If it doesn't smell like roses, a trip to the veterinarian is in order. Home dental preventative care is the best gift you can give your dog. If daily brushing is not performed, plaque will form at the area where the gum meets the tooth. In time, plaque will attract calcium salts and calculus forms.