First time here, I "tested" them by pretty much knowing what my problem was by researching extensively online. In short, I had an engine "boost" leak. The online forums said I would need a "pressure diagnostic test" to make sure which valve/hose was leaking, likely was the diverter valve, and would cost $500 from dealership.
Took my car in, got a call promptly and they said I would need a "pressure diagnostic test" to locate the leak for $90 which can be applied to any repair costs if need be. 1 for 1.
Later, got a call that I had a bad "diverter valve" and replacement was FAR UNDER $500. 2 for 2.
Since then I've had a bunch of repairs done on my stupid Passat and they have always been honest, quick, and reasonably priced.
Only negative is that I wish they would give me a ride home when I drop my car off, I always have to walk!