Tapping a watermelon is just part of testing to see if it's ready to eat. You need to look at the navel to make sure it's brown, not green, also look at the skin color to make sure it's a bit faded. Then it's on to the smell test.. ;)
I know none of this has to do with your metaphor, but you did start out talking about how to test a watermelon. I'm not about to do all those things to check audio cables...just watermelon.