Try to get an allergen/food intolerance history from your child’s care provider. Negative reactions to foods can range from severe anaphylaxis to intermittent diarrhea, and it’s possible that your child may not actually be allergic to a food, but instead, intolerant of it. It’s also possible for children to develop food allergies as they mature.
The best bet is to introduce all new foods slowly into your child’s diet; ideally, one new food every 2-3 days. Sudden or severe reactions to foods will be obvious, but more subtle symptoms like diarrhea, skin rash/eczema, vomiting, respiratory difficulties, bloating, and hives may take time to present.
The common food allergens in the United States are dairy, egg, wheat, soy, peanuts, tree nuts, fish and seafood.