Although the best way to be sure is to check with a vet, peteducation.com has some good tips on diagnosing food allergies in pets.
The diagnosis for food allergies is very straightforward. But due to the fact that many other problems can cause similar symptoms and that many times animals are suffering from more problems than just food allergies, it is very important that all other problems are properly identified and treated prior to undergoing diagnosis for food allergies. Once all other causes have been ruled out or treated, then it is time to perform a food trial.
A food trial consists of feeding a cat a novel food source of protein and carbohydrate for 12 weeks. A novel food source would be a protein and carbohydrate that the animal had never eaten before. An example would be duck and potato, or venison and potato. These diets are available commercially, or could be homemade. This must be the only thing the animal eats for 12 weeks. No treats and nothing from the table. Young growing pets have special dietary needs and a homemade diet that only contains one protein and one carbohydrate with no multivitamin or fatty acid may not be suitable even for only twelve weeks. For kittens undergoing a food trial, a balanced commercial diet like the ones listed above is recommended.
Veterinarians used to recommend that a pet only needed to be placed on a special diet for 3 weeks but new studies show that in dogs, only 26% of those with food allergies responded by day 21. However, the vast majority of the animals responded by 12 weeks. The same may be true in cats, therefore, it is very important to keep the cat on the diet for the entire 12 weeks. If the cat shows a marked reduction or elimination of the symptoms, then the animal is placed back on the original food. This is called 'provocative testing' and is essential to confirm the diagnosis. If the symptoms return after going back on the original diet, the diagnosis of a food allergy is confirmed. If there has been no change in symptoms but a food allergy is still strongly suspected, then another food trial using a different novel food source could be tried.
The only way to accurately diagnose food allergies is with a food trial.
Blood Testing: Many owners and veterinarians attempt to look to other tests to diagnose food allergies. Blood tests can be performed to screen for food allergies. In addition, intradermal skin testing could also be performed. Despite the fact that these tests are routinely performed and used as a diagnostic aid, there is no evidence that blood tests are accurate for the diagnosis of food allergies. The best way to diagnose and treat food allergies you must do a food trial.