Apparently some supernatural being tells Joseph that the only way to detect whether other supernatural beings are good spooks or bad spooks is through the handy-dandy handshake detection test (which by the way is very valuable information for everyday living.)
It just occurred to me thinking about the thread “Epistemic Flaw in Ethical Monotheism” that even if Joe was being completely honest how do we know that the super spook was also. Maybe it was Satan giving Joe the exact opposite advice. In reality good ghosts will be happy to shake your hand and the naughty ones will refuse to shake your hand. This seems much more plausible to me. Detecting authentic supernatural beings is really tricky.

Yes, I wonder how often this very important revelation comes in handy.

Not sure if I've EVER heard of any Mormon having to use it.