Allergy testing is a skin tests that can diagnose allergies to things like mold, pollen, animal fur, insect stings, and foods. With a skin test, the doctor will place a small amount of one substance, or several different substances just under the surface of the skin on the arm or back. You'll feel a little prick when the substances are placed under the skin. After 15 minutes or so, the doctor will look for signs that you're having a reaction. Usually your skin will get red or swollen if you're allergic to something in the test. Another way to test for allergies is to put a patch of the substance on your skin and leave it there for about 2 days, checking the area every day for any sign of a reaction.