The Jende Edge Tester from the Jende Reed Knife Maintenance Kit is used to assess the condition of your reed knife's edge without using your thumb! It also is sentitive enough to monitor the progress of your sharpening by being able to "feel" the difference in refinement levels. It comes with a custom sheath and detailed instructions.

You can find more detailed information in The Jende Book of Sharpening Double Hollow Ground Reed Knives