If your dog is showing symptoms of food allergies, whether it's a rash or diarrhea, consult your vet. They'll ask you questions about your dog's lifestyle and diet to see if they think your dog has an allergy.
Blood and saliva tests are not accurate ways of diagnosing food allergies in dogs. The vet will put your dog on an elimination trial diet to diagnose a food allergy.
Your vet will put your dog on a prescription diet that will need to be strictly followed. Giving your dog treats or scraps is prohibited and will ruin the trial.
These prescribed diets usually consist of a protein source your dog hasn't eaten before, like rabbit or venison, so the vet can tell if your dog's current diet is causing allergies. Your vet may also recommend a change of dog toothpaste and medication to eliminate them as potential allergens.
An elimination trial diet usually lasts 2 to 3 months, but can be shorter. At the end of the trial, your vet may begin slowly adding potential allergens to your dog's diet to pinpoint the exact cause.