Pet food allergy is probably less common than perceived since <5% of dogs and even fewer cats appear to be affected. While there can be some genuine food allergies in dogs and cats, it appears to be more prevalent when a food (a particular protein type to be more specific) is given for long periods. Beef, dairy, chicken, wheat, soy, and lamb constitute major culprits in order contributing ~90% of all the cases. Since food allergy is more long term and immune related, it should not be confused with food intolerance, which is rather immediate and non-immune related. Food elimination test, which is a rather long and expensive process, is probably the best tool to actually determine if your dog or cat is allergic to any food, but blood test and other tools don't appear to be available. Switching the food before too long, alternating the protein sources, etc. should help in preventing from developing or remedying if it hasn't gone too far.