The first step in the diagnosis of a food allergy is to conduct a thorough patient history and a physical examination to identify the most likely mechanism of the disorder (i.e. IgE-mediated or non-IgE-mediated) and identify the suspected food or foods.
The physical examination concentrates on the cutaneous, respiratory and gastrointestinal systems.
The general nutrition status of the patient and any physical signs of underlying non-allergic disorders should be noted. Patients experiencing severe atopic dermatitis or asthma should prompt a more aggressive evaluation.

IgE-mediated food allergy is usually immediate in onset requiring only a small quantity of the food to be ingested enabling rapid identification of allergenic foods. Non-IgE-mediated food allergies are usually slower in onset taking up to 48 hours and larger volumes of food are required, therefore the identification of suspect foods is often difficult.

If symptoms suggest IgE-mediated food allergy, then it is appropriate to perform simple tests such as skin prick tests (SPTs or PSTs) or radioallergosorbent tests (RAST).The skin prick tests identify the foods to which the patient raises IgE. Radioallergosorbent tests are then used to quantify the food specific IgE antibodies and determine the likelihood that they would lead to a clinical reaction.
Suspected foods are then eliminated and challenge procedures are used to confirm the reaction.

If the food allergy is non-IgE-mediated, it may be more appropriate to proceed to a formal and structured elimination process before other tests are performed, but there is an increasing interest for the Atopy Patch Test (APT) in the diagnosis of food allergy in children: patch testing involves prolonged contact of the allergenic extract with intact skin under occlusion for 48 hours.