Both Chris...
To test the edge do just like your Barber showed you, using a wet Thumbnail and drawing the edge straight down using only the weight of the blade feeling for Rough (chips) Smooth (dull) and the edge to dig into the nail (Sharp)
By running the side of the bevel over the thumbnail causes a slight bump (Flex) and that will expose cracks, and also pits in the side of the bevel..