I'll agree that it is not a definitive test, but it sure helped me recently. I got a #6 dutch oven, and it was described as having no cracks. I looked the oven over for a minute or so when I recieved it, and saw nothing amiss (in fact, it was Beautiful). When I tapped it, though, you could sure hear the buzzing or grating of the edges of the crack. It was a very tight hairline, about an inch long on the lid. I returned it with no problems.
I think that it couldn't hurt to give it a rap before buying, if you have it in hand.