They can do a scratch test but I do not believe it tests for food allergies. There is some controversy over the scratch test versus the blood test. It’s been years since I researched this so memory is vague as to why there was a disagreement over it.
Alot of people who have allergic dogs recommend that you take your dog to a veterinary dermatologist as they are the “go to” guys for allergies.
Years ago we had our border collie tested. It was a simple blood test and it cost around $100. She was allergic to grass pollen, dust and mold. We started to give her allergy shots. They did the trick and the dog never had problems after that. I think we gave her the shots for about 3 years.
The allergy shots do not work for every dog but if the dog has more than a seasonal allergy that cannot be controlled with an antihistamine the allergy shots are worth a try. Alot of vets will just put a dog on steroids instead of getting to the bottom of the problem and steroids have alot of nasty side effects and are not really a good option long term.
For food the vets use an elimination diet. You can google it. I believe they start out with one simple food and you add different things until you get a reaction out of the dog. It is time consuming and you have to follow it to the letter but if you have a dog with a severe food allergy it is worth it to get to the bottom of the problem.
We had a cat once that had allergies and yes, it was unbelievably expensive. But in the end, the decision was to put him on steroid injections regularly and it worked like a charm. No more biting at his fur and ripping it out in patches. Was a much prettier cat after that. LOL