To genuinely know if one's dog is allergic to *anything* is to take it to a veterinary specialty school and have it tested for allergies.... no general practitioner vet can say if the dog is allergic to anything; otherwise it is speculation or at best, a guess. They are not a dermatologist , nor the testing lab ( or results ).......
If one would ask a vet dermatologiest what the percentage of dogs tested are allergic to anything, it'd be very low .....
If someone cannot afford the hundreds of dollars for the allergy testing, then process of elimination will take a very long time... detoxing from one food to another can take up to **six** months. And even then , it may be a simple matter such as dry skin from a no to low humidity season or something as odd as a new collar to a new pet bed.
fwiw....
Merry Christmas to all ~