If you tap the prop with a stick, it should give you a good solid tone. (Metal prop of course.) I've been told if the prop has a fracture, it will buzz a little. Never heard the buzz, but I've also never had a fractured prop either. More than one older instructor has told me this, so it's either a well known thing, or some urban legend that seems to have been passed over the years because prop fractures are so rare.
Anyone got any other "tips" of that nature that aren't in the POH? (Such as, in the early 177/Cardinal, always check the fuel vents. They can jam the aileron if they shift.)