Most of the people on this forum have experience with life-threatening reactions to foods, either in ourselves or in our children.
So our journeys begin with trips to the emergency room (or just reactions that we treated ourselves and got lucky with in our ignorance)-- breathing difficulty or widespread swelling and immediate hives are fairly common, and symptoms of shock are also fairly common.
My flippant answer to your question (quite a common one), therefore, is usually-- "the hard way." Sometimes it can be hard to pinpoint which thing was the trigger, but believe me, there is generally NO question that it's an allergy, probably to food. Very tiny amounts of the offender may lead to symptoms like eczema or rashiness, but those symptoms are often more related to environmental allergies, or occasionally to celiac disease.
My advice to you?
See a board certified allergist with experience in food allergy in particular. Start keeping track of WHEN your symptoms begin after eating. Be sure to bring your food journal with you; a good clinician can definitely figure out a LOT with that kind of data!
Good luck!