The only way to reliably tell if your dog is allergic/intolerant to a food is by using the elimination diet. This is where you provide one new protein source and one new carb source for your dog to eat, and only eat this, for 8 to 12 weeks. There should be no treats or table scraps while on the elimination diet.
CAUTION: If you have a special needs dog, a puppy, or a pregnant or lactating dog, the elimination diet may not be advisable, as your dog may not be getting all the necessary nutrients during this time period. Consult with your vet before any change of diet. If your dog food allergy symptoms disappear (this will not happen for a couple of weeks), then it may be a food allergy. To confirm this, after the 12 week elimination diet is completed, you can either go back to the old diet or try introducing foods one at a time and watch for reactions. The most common foods to provoke allergies in dogs are wheat, corn, chicken, dairy, and soy – although anything can be the culprit.
If you don’t want to, or don’t feel comfortable feeding your dog as described above, then the best food for dogs with allergies would be a quality hypoallergenic dog food, which is packed full of nutrients your dog needs. Lack of quality nutrients can actually frazzle an immune system, causing it to always be on the attack and mimic allergies.
A hypoallergenic dog food has a limited number of ingredients which helps to reduce the possible allergens your dog is exposed to. Some of these foods will also use ingredients your dog has probably never had, such as duck, venison, barley, and sweet potatoes, and thus have no adverse reactions to. Many people see a great improvements in their dogs allergies just by switching to hypoallergenic dog food. While not the most common reason for an allergy, dog food is by far the easiest to control for testing. Two of the best hypoallergenic dog foods are Wellness Simple Solutions and Natural Balance LID.
Feeding your dogs with the right food will stop your dog itching.