Not everyone who reacts to certain foods has an allergy, however. They may have a food intolerance that does not involve the immune system (eg. lactose intolerance, non-celiac gluten sensitivity or sulfite sensitivity).
Lactose intolerance (owing to a deficiency in lactase, an enzyme that breaks down milk sugar), is fairly simple to diagnose via a blood test, breath test or stool acidity test. However there are no validated diagnostic tests for non-celiac gluten sensitivity.