Start with history. Sit down and write out your symptoms, when they appear after eating, how they progress, when they resolve. List all the foods you ate before symptoms appeared. If a processed food, save the ingredient list. Write down what medications you tried & how they worked. Take this information to a board-certified allergist. Together you can decide on relevant skin or blood food allergy tests.
See an allergist. After the allergist takes a detailed history (ie. What foods you are concerned about and what reactions you experienced) they will decide if skin testing or blood testing is the best way way to start.