Applied Kinesiology and Vega skin conductivity testing can be helpful in testing for food allergies and what types of vitamins to take but is not good for pinpointing health problems.
In one study applied kinesiology was found to be 90% correct in identifying food allergies.
So it is fairly good at testing for food allergies but I would warn against relying on this test for determining specific health problems. Here a questionable on-line statement about it: "We are often able to check out potential stressors to the body such as candida, beneficial bacteria deficiency, digestive enzyme deficiency, blood sugar handling problems and other imbalances. The vials are placed on the body one by one. The frequencies from the vials may alter the response of the muscle being tested, which enables us to find out key imbalances that need to be treated."
OK so let's propose that we want to test for candida. What substance do we put in the vial for the test subject to hold? If we put some candida fungus in there then theoretically everyone should test weaker because all bodies should reject it as a parasitic fungus. But maybe some people with candida have bodies so full of it that putting a sample of it close to their thyroid won't provoke any negative reaction. So that's not a good way to test for it. If we put some anti-fungal medicine in the vial then probably everyone would react negatively to it because most medicine in itself is a type of poison albeit more venomous to the target pathogen than to the human body. So that's not a good test. Hmm, what else could we use? Maybe a harmless homeopathic remedy for candida. Well no, because the body is not "intelligent" to try to let you know to use that remedy because it has a candida infection. So in reality there is no way to use muscle or skin testing to test for the presence of candida or other health problems.