If your dog or cat seems to be scratching and licking frequently, you should take your pet to a veterinarian in order to determine if it actually is an allergy. Other diseases can cause similar signs, and proper diagnosis is the key to proper treatment.
After observing your pet's symptoms, your veterinarian may be able to make a preliminary diagnosis based on some obvious cause/effect deductions. For example, if every year your dog begins showing symptoms in the fall as the ragweed flowers, he may be allergic to ragweed. The veterinarian will then recommend a treatment plan that could include fatty acids, biotin, antihistamines and, in some cases, steroids.
You can help confirm the doctor's diagnosis at home by noting your pet's response to the allergy treatment your veterinarian has chosen. If all goes well, your pet should be back to normal until the following year. Your veterinarian will keep note of the allergy in your pet's records and probably repeat the same treatment when the allergy season returns.
But if the symptoms don't at first point to an obvious allergen, or if the treatment recommended doesn't seem to have an effect, your veterinarian may recommend a more precise allergy test to make a diagnosis. An allergy test is the most accurate way to pinpoint the cause of his allergies. This is done by either by taking a blood sample or via tests right on the skin. There are two types of blood tests that can be used, RAST tests (radioallergosorbent) and ELISA tests (enzyme-linked immunosorbent assay). Both require a blood sample sent to a laboratory for testing. While mostly reliable for detecting allergies to inhaled substances (i.e. pollen), these tests are less accurate for diagnosing contact or food allergies.
The other option is intradermal skin testing. For this procedure, your veterinarian shaves away a patch of your pet's coat and injects a small amount multiple allergens into the skin in two neat rows. After a few hours the injection area is checked for any reactions, such as a reddening or raising of the skin. Those allergens causing reactions can be included in an allergen mixture prepared by a laboratory. Multiple hyposensitizing injections can then be given over a course of months to desensitize the pet to those allergens.