When your dog develops food allergies, your veterinarian will often recommend a dog food trial. These are usually done under veterinarian supervision but can also be done at home, if you know what you're doing.
Finding a Novel Protein
Dogs can be allergic to almost anything in their food, including protein sources such as chicken or beef, vegetables, grains and preservatives. Thus, to determine the cause of the allergies, it's important to eliminate everything from your dog's diet and find something he's never eaten before.
There are hypoallergenic commercial diets that consist of novel protein and grain sources, or you could create the diet yourself with homemade ingredients. It just must be made up of a novel protein and grain source, such as rabbit and potato or duck and potato. If your dog has a sensitive stomach, this will be more difficult because you may have tried these varieties. You'll have to seek out a bison or kangaroo based food.
Do's and Don'ts
There is only one do: Do feed your dog the new food that you've found with the novel protein and grain. You have to feed this food for 12 weeks to be able to ensure that all the old allergens are cleansed from your dog's body. The don't list is much longer: Don't feed your dog anything else at all.
Don't feed your dog treats, bully sticks, rawhides, pig ears or any other snack you usually give. Don't give flavored medications or supplements. Don't give your dog flavored toothpaste or toys like Nylabones. Don't give your dog any table scraps, including bread or cheese when trying to give a pill.
Don't allow your dog to roam around anywhere that he might pick up a stray scrap. Lock the trash can in a different room. If there are any hiccups in your food trial, you have to start the 12 weeks from scratch. Make sure everyone in the family understands that. Consider keeping him in another room during meal prep and eating times in case someone drops something.
If you are used to giving your dog treats, get wet food of the same variety as the kibble you're feeding, cut it up and back it on a cookie sheet. This can provide a hypoallergenic treat.
Adding Additional Ingredients
Once your dog has been on this diet for 12 weeks and the allergy symptoms have ceased, you can begin to add ingredients back into your dog's diet. However, you must do it slowly, ensuring only one or two ingredients get added back in at the same time. If you add too many new things, you won't know what's really bothering your dog.
Start with a food that has only one new protein and grain, such as chicken and rice. Feed this for a few weeks before adding a new protein/grain/vegetable combination. If the symptoms come back, you know you have found an allergen. Since your dog can be allergic to several things, keep a list, and continue not feeding treats, table scraps etc until you have experimented with all potential ingredients.
Though food trials take a long time, it's really the only way to diagnose food allergies. Blood tests have been shown to be ineffective, so if you want to relieve your dog's symptoms, you have to try a food trial.