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I dislike the thumbnail test. It has the same effect on me as someone putting their nails on a chalkboard (but not dragging them down). I just sweep it about a mm above my arm. If it snags some hairs...

My recommendation would be this:

(1) Ebay Solingen razor (or if you have more time, straight razor, and look at all the names off the "good razors" list + razors with Solingen on the tang)
(2)...

If anyone accepts, you should make him wax and buff your car. Tell him it's part of his training.

Very good point, I absolutely agree that a sharp blade is necessary to compare against... unless you have plenty of body hair and a high clotting factor. ;)

I can tell a sharp knife on first...

If you create a sharp blade through practice and a great deal of time and experimentation, trial and error, and USE that blade, hold it, maneuver it, polish it, and just look at it for a long time...