If you are looking at people like Airbus, BAe, CAA they normally want somebody who has graduated from one of the main TP schools (EPNER or ETPS in Europe). Ideally such people also have a few thousand hours in the normal operating environment too.
ETPS usually looks for at least 750hrs P1, 2 tours as captain, and an "above average" or "excellent" rating. In practice few of their students have less than 3000 hours.
Piper tend to use engineers with considerable experience as FTEs (Flight test engineers), and have self-improved to CPL level as pilots. Not sure about Cessna.
Most smaller manufacturers (or organisations like EAA and PFA) use whoever they can get - usually a mix of retired military test pilots, or airworthiness engineers with a lot of flying experience.
At the bottom end of the weight scale, the USUA tend to use ultralight instructors with no formal test flying experience, the BMAA publish a formal syllabus at <A HREF="http://www.avnet.co.uk/bmaa/014.pdf" TARGET="_blank">http://www.avnet.co.uk/bmaa/014.pdf</A> and so far as I know are just about the only people training light aircraft TPs themselves.
In Germany there's a multi-tier qualification system of class 2 and class 1 test pilots, partly trained in-house, and partly by the big schools.
Personally (and I use TPs a lot, although don't claim to be one myself - I only do a little lower level testing as a pilot) I look for somebody with a good degree of technical understanding of their aircraft (degree level, not ATPL level), a very very high ability to identify and record in detail what an aeroplane is doing, an obsession with planning and safety, and enough flying ability that I can reasonably expect to get the aeroplane back. The last is frankly the least difficult skill to find. Oh yes, and considerable experience in a wide range of types - somebody with significant hours in only a couple of types is not equipped to compare characteristics well.
G