My eldest Shih Tzu has allergies & I had him tested several years ago. He also has chronic ear infections.
It was just a simple blood draw that I recall. It was sent in & I received a list of environmental allergies (dust, pollen, grasses, trees, etc.) fabrics, foods, etc. They gave me quite a large list of dog foods that were "safe" for him to eat. He has SO many allergies that this is wonderful to have. I believe I paid around $250 for the test but it was well worth it IMHO. I tried the allergy injections, where you give them to them & it hopefully builds up their immunity to the allergens but after about 11/2 years I saw very little difference so I stopped them. The vet said it was kind of a long shot them working but like I said he has SO many. Fortunately, I have a very sweet dog that would lay down & roll over when it was time for his injections. Bless his little heart.