An allergist will ask you detailed questions about your child's symptoms. Then she may give him a skin or blood test to determine whether his symptoms are due to an immune reaction. If the skin test produces a rash or the blood test shows that he has antibodies to the food, there's a good chance that he's allergic. If the tests are negative, your child's probably suffering from a food intolerance. The shots and immunotherapy methods that treat other allergies such as hay fever don't work with food allergies, so avoiding the food is the key to preventing a reaction.
Avoidance is also the best strategy in the case of a food sensitivity.
Although a food sensitivity won't cause a life-threatening reaction, it can make your child very uncomfortable. Unfortunately, avoiding a particular food is often trickier than it sounds. Foods can show up in unlikely places and even a little bit of something may be enough to trigger a severe reaction. You'll have to become vigilant about reading food labels, knowing which ingredients to avoid, and asking about ingredients in restaurant dishes. For example, cows' milk and other dairy products show up in canned and processed foods as casein, whey, lactalbumin, caramel color, and nougat, among others.