Elimination and paying attention lol she has bad poo, screaming and face rash when i eat something she cant have, so i eliminate it from my diet then re try it a week or so later if we have the reaction then. then we know iykwim
Eggs and soy took us a while we had about 4 reactions on eggs before i gathered it was the problem (esp since gluten was always pressent too) and soy was 2-3 (Ironicly it was curry sauce from the chippy which confirmed us) so i cut both and tried them isolated and it caused a reaction
Nuts was day after i had peanut butter in the night she had a reaction but i was already egg, milk, soy free and knew there were no slips