We figured out my son had food allergies when he had an allergic reaction to dairy simply by touching his skin. Later, we did a skin allergy test that gave us an idea of all his allergens. We are currently working on overcoming his allergies, so this is an ongoing process.
We didn’t use any websites. We went straight to a food allergist, who explained his different food allergies. We’ve also gone through trial and error, figuring out which other foods he’s allergic to as we introduce new foods.