Did Mike pick that up from one of the old machinist handbooks? I read about the fingernail test a while back in an article on tool wear and it said the same thing, your finger can catch edge damage way before your eyes pick up on it in the cut. Something about how a nail drags across those tiny chips and cracks that you just can't feel with your fingertip. I started doing it on our inserts and it saved me a couple times when the finish still looked fine but the edge was already going. The catch is you gotta be careful, that edge will slice you open if you get too casual with it.