There are many methods of allergy testing.
Skin tests are the most common.
Specific methods vary. The skin is observed closely for signs of a reaction, usually swelling and redness of the site. Results are usually obtained within about 20 minutes, and several suspected allergens can be tested at the same time. Because food allergies are often affected by what the person thinks they might be allergic to, a double-blind test may be advised. This test requires several sessions if more than one substance is under investigation.