It can be very hard. You need to figure out how long it takes you to have a reaction. My son and I are both celiac. It takes him two days to notice a reaction and I usually notice the next morning. Then it can be anything I ate the day before. I keep track with a food/symptom journal. I keep my diet simple and try to only add one new thing per week. When I am reacting I keep eliminating and hoping for an improvement. I usually show an improvement by the next day and again, it takes my son a couple of days. It helps that we can compare with each other. It helps to check with manufacturers for some change, for shared facilities, etc. I also talk with my farmers at the market about their farming practices. I use that information to decide which things to eliminate first. It can be really challenging, but worth every bit of it when you finally succeed. My son is now running when before he couldn't walk around the block without feeling sick.