I would have thought that a nerve test is to see if the nervous system works properly; usually a particular nerve
I'm not too sure on the exact details on what it would entail but I doubt that it will be too much of a problem (The test itself) as problems with the nervous system has proven to be a total pain; such as the disability in feeling things, the reflex system (Where you quickly move your hand away from a stimulus which may cause pain - such as a flame)
I had this sort of test done years back and it was done by stimulating the nerve to test the response (by using electrodes). It's not painful and it is to check that the nerves are working correctly.