Evoked potential tests--
--differ from electromyography (EMG) and peripheral nerve conduction studies in that they measure the processing rate for sensory modes (sight, hearing, mechanical touch--VEP, AEP, and SSEP, respectively) in the central nervous system. As such, they are normally used to help diagnose central nervous system (brain, spinal cord) problems rather than peripheral ones. They are sometimes used to check for possible demyelination, as in transverse myelitis or multiple sclerosis.
It is possible to have damage both to peripheral nerves and the central nervous system, however, and B12 deficiency is certainly on the list of candidates for that.
The current gold standard for discovering small-fiber neuropathy is the skin biopsy to enumerate intraepidermal nerve fiber density and condition, although there are other tests, such as quantitative sensory testing and certain autonomic tests that are also sometimes tried.