Itching is the most common sign of food allergy in dogs

A lot of people believe that their dog has a “sensitive stomach”. What a sensitive stomach actually is, is a lack of healthy bacteria in the gut which allow the dog to change brands and proteins frequently without digestive upset. Just like a human a dog should be able to eat any type of dog food (unless they have a known allergy to a certain protein or carb etc) and not have digestive upset at all.
The stomach becoming unable to adjust to diet change, is due to eating the same food for many years or even months.
My dog used to be the same way. If i did have to change his food and did it too fast, he would get diaherra for a few days. Since adding digestive supplements to his diet and a probiotic (i use kefir, you can use plain yogurt too) I have been able to switch what brand he eats every bag without blending his food and he has no diaherra at all. Quite a few people on this site are actually able to do that with their dogs too.

One of the main symptoms of a food allergy in dogs is itchy skin. This can manifest as

Evening primrose oil, an essential oil,has active anti-inflammatory and anti-itch properties. You can apply l directly to your dog’s skin to treat dry or itchy skin issues. Likewise, giving your dog evening primrose oil by mouth helps calm allergies and boosts the immune system – and it only takes a few drops on her food or in her water to do the trick.

Paw Print Genetics - Why is my dog so itchy? Part I: Food Allergies

Yet food is the first to be blamed whenever a dog shows any sign of an allergic reaction — like itchy skin. Some animals have many allergies. It would not be particularly unusual for an animal with a food or inhalant allergy
Flea
to also be allergic to flea bites, especially considering that flea bite allergy is extremely common among pets. Because allergies add to each other, it is possible that a food-allergic dog will not itch if its fleas are controlled. Since new technology has made flea control safe and convenient, it is especially important (and no longer difficult) to see that fleas are not complicating a pet’s itching problem.

Dog Food Allergies: What Causes Your Dog's Itching

I recently switched my dogs to Taste of the Wild, after hearing so many good things about it. They had been on Eagle Pack, then Holistic Select, prior to the formula changes but did not do well after whatever changes were made.
We have not quite finished the first bag of Taste of the Wild and I just put it in the trash. One of my dogs developed food allergy symptoms, licking paws, scratching sides and belly, behind elbows and worst of all, painful anal area. The other one developed milder licking and itching symptoms, and became very lethargic. I have a normally very active breed.
They are currently on home cooked food, so I can control the ingredients while searching for yet another food that my dogs could actually eat.
Hate to go back to full time home-cooked for them, but it is becoming so hard to find a dog food that can be trusted to not only have, but maintain, quality ingredients and processing, that I may just give up.

Best Hypoallergenic Dog Foods - Dog Food Advisor