This kind of test does typically work with ceramics. The higher the note produced when tapped, the higher the temerature to which it was fired. Now, this is a very subjective test, but it pretty much hold true. If it really thuds, it's probably cracked.
As for knives, there are so many different steels, coatings, thicknesses, etc. out there that I doubt this same test would mean too much. However, if I had a blade that was uncoated, of average thickness, and it made a thud when I tapped it, it might make me look more closely at it for other signs. I may try this later and see what I see.
Of course, I just tried it on my Sebenza and it went "clunk". This is probably due to the fact that the bushings in the pivot act to damper the vibrations. This test probably won't give any kind of reliable results on folders. Edit: This test probably won't give reliable results on any knife for the above mentioned reasons, but folders in particular have the problem of being a broken knife as compared to a fixed blade, so I wouldn't expect this test to... aw hell. It's a neat idea, just don't rely on it.
-Al-
