While a skin biopsy that shows reduced intraepidermal nerve fiber density and/or damage is the current gold standard for confirming a small-fiber syndrome, there are other tests that can point to it--most notably, abnormal quantitative sensory testing (which can reveal abnormal rsponses to changes in temperature), and several different autonomic tests (such as sudomotor axon reflex testing that can reveal abnormalities in sweat response, which is controlled by the autonomic small fibers).

The good reason for the skin biopsy is it actually looks at the condition of the fibers themselves. While the other tests can reveal abnormalities, they can often be normal in someone who will show up with an abnormal skin biopsy, as autonomic damage does not happen in all small-fiber neuropathies, and sometimes the fibers that subsume temperature sensation can be basically intact while the small nerve fibers that subsume pain are selectively damaged (which would render qualitative sensory testing equivocal at best). Moreover, the damage can be spotty or patchy.