Prick Test
This test involves a needle pricking or scratching the skin, after which a solution with a possible allergen is introduced into the raw area. Developing a rash or a weal on this spot indicates that the person is allergic to the substance applied, thereby tagging it as an allergen. This is one of the most frequently used tests due to the accuracy and speed of the results. Despite the name and method, prick tests are painless and safe.
Intradermal Test
In essence, this test is the same as the prick test. The only difference between the two is that the allergen solution is injected into the skin. This test is usually given when the doctor suspects a false-positive reaction during a prick test.
Blood Test
If these simpler test are inconclusive, the pediatrician can order a blood test. After extracting blood from the patient who had been exposed to the suspect allergen, laboratory tests determine the level of immunoglobulin, the antibody that the body makes when exposed to allergens. A high level usually indicates the presence of an allergy.
http://kidshealth.org/parent/medical/allergies/allergy.html