Neuropathy, whether stemming from disruption of one or more peripheral nerves (actual "peripheral" neuropathy) or from disruption of nerves in the brain or spinal cord (which can happen through conditions like multiple sclerosis or mechanical compression), is a very hard conditini to pin down, there are over two hundred KNOWN causes of nerve damage--not even getting into those medical science has not yet identified. Some of these are secondary to other well-known condtiions, such as diabetes, thyroid dysfunction, or autoimmunities; some of these are primary conditionios of damaged nerve. In any case, since neuropathic symptoms can be produced anywhere in the body by problems of the brain and spinal cord, problems of the peripheral nerves, or both, the diagnostic process can be long, costly, and very often fruitless--at least a quarter of all people with nerve dysfunction wind up being labelled "idiopathic".
That being said, there are diagnostic protocols out there that should be followed. These can get lengthy and expensiive in many cases, and not all insurances cover all tests; one often has to decide just how far one wants to go in testing if one does not discover the cause of one's symptoms quickly.
Some sites that help provide source material for people to talk about with their physicians:
This is the diagnostics entry point for the huge neuromuscular conditions database at Washington University of St. Louis' medical website. It is written primarily in outline form, and mostly for medical professionals, but it is extraordinarily comprehensive; going through the links and sub links reveals a wealth of info on neuropathy (and other neuromuscular conditions). There are parts of each sub-section that are titled "Laboratory", indicating what tests might reveal that particular condition, as well as listings of signs/symptoms that might point to that particular diagnosis.
The famous Poncelet protocols. The paper is titled "An Algorithm for the Evaluation of Peripheral Neuropathy". Outside of neurology textbooks, this article has become the "gold-standard" guidleine for many physicians and neuros for investigating the causes of potential neuropathies. The flow charts are particularly instructive (and good to show ingorant physicians).
One of the best articles I know written about small-fiber neuropathies, which can be difficult to diagnose, as "standard" tests for neuropathy often come up "normal" and fail to reveal them (and meanwhile patients suffer). The reference list is excellent--it includes many of the seminal papers on skin biopsy, autonomic testing, and small-fiber vasculitis and other causes of small-fiber dysfunction (these can themselves be investigated if one wants).
A small but dense clinical application paper about laboratory tests to investigate causes of peripheral neuropathy. Dr. Latov heads the Cornell Weill Center for Peripheral Neuropathy in New York City, and he and his staff may be the world's leading experts on immune-mediated neuropathies (not that they're slouches in other realms, either). (All docs there are both researchers and clinicians--and, I can tell you from personal experience, and informed patient can have great discussions with them.)
Another Latov paper, this one from the neuropathy.org website (the site of the Neuropathy Association), that specifically discussed the diagnosis of immune-mediated neuropathies, a category that most non-specialist physicians are not well-versed in.
Another good paper--this one form Dr. Alan Berger and his colleagues at Jacksonville Shands. It provides a good overview of diagnosis and treatment options for various kinds of neuropathies, and suggests a particular categorizing method to understand them.
And, of course:
This is a wonderful spreadsheet series that was put together by our very own LizaJane, with input from a number of other people who frequent Neurotalk, and designed to be as comprehensive a listing of possible tests for neurological conditionos, both central and peripheral, as our minds could collectively come up with. Broken down into symptomological sections, and with space to track test results over time (to look for patterns), it's wonderful for suggesting avenues to physicians and for noticing changes that might provide diagnostic clues.