There is not one definitive way to diagnose if your dog has dog food allergies. There are a couple of factors to look at.
Dogs that are intolerant to certain foods may have physical problems such as diarrhea, vomiting, gassiness, or bloating of the stomach.
Food allergies are caused by an immune system reacting inappropriately to the present food that is being ingested. This is when we try to find the offending food to decrease or eliminate the problem.
Dogs also suffer other symptoms due to actual allergies rather than intolerance. Itchy skin is the most common symptom of a dog who is allergic to the food that he is presently on.
This itchiness is usually noticed on the dog's sides, tail, butt, paws, face and ears. The incessant itching, scratching and licking is sometimes as irritating to us as it must by for your dog.
The allergy needs to be found and eliminated quickly, so the incessent licking and scratching doesn't go on to cause open wounds that can get infected.
An obvious food allergy will show up soon after switching or changing your dog's food. Removing the recently introduced new food should take care of this problem. Adding only one new food at a time over a period of many weeks will help define which foods your dog can tolerate.
If your dog's allergies suddenly start even though no new foods have been introduced, then it may become more difficult to find the cause of the allergy.
This is the immune system reacting to a sudden sensitivity to the allergen, and is harder to figure out what has caused this problem. In this case you will have to start from scratch to try and eliminate this dog food allergy. The present diet must be completely stopped.
Although we all know by now that we are supposed to slowly introduce new foods slowly, and one at a time, in this case switching suddenly is necessary.
Put your dog on a diet consisting of foods to which he has never previously eaten.
If you have fed a chicken based dog food, then you can try another protein based kibble. Many foods for dogs with allergies are completely grain free, because grains are a known allergy causing ingredient for many dogs.
After starting a new diet, it may take several weeks for the allergy to slowly stop. Introduce a new diet consisting of a minimum essential ingredients, but making sure the diet is also nutritionally complete.
The fewer the better.
There are many excellent high quality, chemical free, grain free dog foods available which have just the very basic ingredients necessary for optimum health.
Always consult a vet if the symptoms are sudden and severe.
There are also many other types of allergies that are not food related, such as infestation of parasites and fleas, environmental or even seasonal allergies.
These should be eliminated as the cause of your dog's problems and are treated differently than dog food allergies.
It is not always a quick process, but if you can relieve your dog of discomfort, it is worth time spent investigating.
