There are a lot of different ways to test for allergies. Most conventional allergists will only use the RAST or skin prick/skin patch test, which will only show IgE allergies (see Types of Allergies). There are alternative tests which you can get through a natural practitioner, like an ND, or sometimes directly through the lab that does the test. Two of these are the ELISA and the ALCAT. There is also testing which is considered more alternative- such as muscle-testing (kinesiology) and pendulum/crystal testing. I will try to give a brief description of each test. But before trying any of these tests, you should keep in mind that NONE are 100% accurate. Some are better than others, depending on the type of allergy you have.
RAST (radioallergosorbert test) - this is a blood test usually done by an allergist. Your blood is drawn and tested; no allergens are placed in/on your body. This test measures the amount of IgE that reacts with a specific allergen. An allergist will choose which foods or environmental allergens are tested. Good for people with eczema or reactive skin as opposed to skin tests. Good for environmental allergies, or IgE food allergies. Not very accurate for babies.
Skin tests - There are a few different types, but the most common is the skin prick test. The skin is scratched with a little needle, then an allergen is rubbed onto the scratch. The amount of swelling on and around the spot is measured to show an allergic reaction. This test also shows only IgE allergies; it will NOT show food intolerances. Not very accurate for babies.
Patch testing - I haven't actually been able to locate a doctor who uses this test, but I'm told that some do. From my understanding, allergens are placed on discs which are taped to the body and left for a period of time (48 hours?) then checked for swelling and redness. This test is said to work for both IgE and IgG allergies.
ELISA (enzyme-linked immunosorbert assay) - This is a blood test done to check for delayed-reaction allergies or intolerances. Most will NOT show IgE allergies, although some labs do a combined IgG and IgE. There are many varieties of this test, and just like all the others it is not 100% accurate. Can usually test for hundreds of foods with one blood draw. Accurate (or so I'm told) on babies of any age.
ALCAT - This is another blood test that is for finding delayed-reaction allergies; will NOT show IgE allergies. Measures leukocyte cellular reactivity in whole blood. There is some debate as to whether you need previous exposure to a food before it will show up on the test accurately. This is one of the few tests we didn't try, so I'm not as familiar with how it all works.
Muscle testing (applied kinesiology) - This test is done by alternative practitioners, and is used for many things besides allergies. The basic idea of this is that you can hold a food or chemical in your hand, the practitioner will press down on your arms and determine whether the arm holding the substance is weaker- indicating that it is unhealthy for your body. We have also not used this test, so I welcome comments or additional information about it.
Crystal/pendulum testing - This test is easy to do yourself (and free!). It involves placing the questionable substance in one hand, holding your pendulum over that wrist, and watching the direction or pattern of how it swings. The results are not the same for everyone- you have to test your crystal/pendulum to see what means "yes" and what means "no" for you personally. If you get a "no" reading, then you know that substance is not healthy for you. Here's a cool video of crystal testing in action.