Recognize first... Based on clinical history and suspicion. Then test correlative to the history. If any positives show up, you should avoid and re-test at frequent intervals to determine if your child has outgrown the allergy. And please do not underestimate food allergies, always carry an epi injector if symptoms are anything beyond eczema flaring.
Have Patience... Feeding an allergic child takes planning, self-education, creativity...And patience! but if you practice careful avoidance, your child may actually outgrow food allergies. On the other hand, when they eat foods they're allergic to, even if the consequences seem minor, the allergies are more likely to last--and possibly to become more severe. So, how do you spell relief? A-v-o-i-d-a-n-c-e!
Avoidance. For food allergies, avoidance is the best option. However, if the food is a nutritionally important food such as milk or wheat, it may be best to be tested to make sure. An allergist can help decide if you child has a food allergy, give tips on avoidance, prescribe an epi pen for severe allergies, and decide when to test if the food allergy is decreasing since some are outgrown.
Diagnosis. Before you can avoid; need to determine exactly what your child is allergic to. Then a nutritionally sound diet can be determined. Avoidance is necessay but only to real allergens. Withholding nutritionally good foods when there is no need to may compromise the child's nutrition and make preparing foods more of a problems then it needs to be.
Avoidance. I often test for allergies on foods for infants with eczema (skin pb) or recurrent sinus infections. Allergy testing can identify unsuspected food items & give an idea of intensity. Low intesity items can often be eaten on a rotation diet. Major food reactions are rare, but deserve rigerous avoidance & preparation. (read lables, epipen) many kids grow more tolerant of trouble foods with age.
Avoid suspect foods. Babies in their first few months of eating solid foods have a variety of baby foods to choose from. Use each new flavor for 3 days, to see if there is any rash, fussiness, or undesirable changes in the poop. If some mild symptoms occur, the baby can skip that flavor for now, but consider trying it again 6 months later. If the symptoms occur again, then the child can avoid that food item.