We use a blood test that measures food allergies that are IgE (immediate response) and IgG (delayed onset).  Everyone knows when they are immediately allergic to a food but it is the delayed onset allergies that are harder to pinpoint. 

Food allergies have been associated with a range of medical conditions, and depending on the total antigenic load (how many allergens are activated at one time), can range from annoying to severe: Can affect mood and behavior (especially in children), Asthma, Migraines, Sinusitis, Urticaria (hives), Ear infections, Arthritis, Eczema, Colitis.