The best approach is to search for the food intolerance test on Google or Amazon, to see description and reviews, and to pick what you like, what looks better to you. Read the test results carefully, and begin experimenting by excluding some foods (but keep in mind that when you eliminate some foods, it still takes time, usually weeks, to verify assumptions, i.e., to see if this is the food that causes a delayed reaction or not).
Here are some providers you could choose from: