TW's method is pretty much the way I do it with my thumb lightly on the extractors/ejectors to dampen any vibration there, but I tap on the rib itself. If the rib is loose, you'll get a dull click along with the ring indicating a separated rib at that point. Barrels held & ringing like a church bell will undoubtedly astound and amaze many a potential buyer or owner. But in reality it cannot tell you anything but the most generously loose rib. It's a real rarity that there isn't at least some pinhole(s) or small loose section somewhere along those lengths of solder joint. I can recall very very few over the years that didn't 'leak' while refinishing and that includes all makes and qualities.
