Living in Florida, I get to see more dermatologic cases than are seen in other parts of the country. What other vets see in a year, we see in a week.
In cats, allergies to food more typically show symptoms that affect the digestive system...vomiting is most common and then soft stool to diarrhea, and then perhaps in the skin. Other types of allergens will react more in the skin, causing itching that results in hair loss, inflamed skin and secondary bacterial infections. Actual hot spots on the face are not typical.
Not knowing exactly where the hot spots are located on the face, I would first check the ears and make sure something as simple as ear mites aren't causing the cat to scratch excessively at them and causing the hot spots. Another possibility is an actual contact allergy, such as to plastic food and water dishes. Switching to stainless steel dishes is so simple and easy that it simply has to be tried.
We've done quite a few allergy tests in our hospital, although mostly dogs. The most common food allergies are corn, wheat, beef and chicken. Most cats can't digest lactose as adults, so we don't recommend dairy products to any cat having allergies or not. Switching to a limited ingredient diet (venison, duck, rabbit - not seafood in cats) may give you the desired results, if a food allergy is the cause. Prescription diets (Hill's, Purina, Royal Canin, etc) will be better than anything obtained over-the-counter. Using a hydrolysed protein diet will also work (Hill's Z/D). And to make a proper food allergy trial successful, you must completely eliminate all other foods, including treats, table scraps, etc and feed exclusively the new diet for about 2-3 months before saying a food allergy is present or not. You also can't use any other allergy treatments in conjunction with the diet change.
A problem arises as only about 30% of allergies are to something in the diet. The other 70% are to things in the environment, such as pollens (grass, tree, flower), smoke, dust mites, other animals, dyes, certain cloth, etc, etc, etc. Without testing, it can be very difficult to determine what a pet is really allergic to, and often, you find out it's something you can't keep away from your pet. And those "allergy shots" made up by the testing companies are a waste of money.
We've found that in the majority of cases, if the food trial doesn't work 100%, having to administer corticosteroids periodically to a cat is the best way to go. Cats tolerate corticosteroids much better than people or even dogs, and in many cats, they only need an injection once to three times a year.
