To test a honed razor, a barber should:

Testing a honed razor by lightly passing it over a moistened thumbnail is a reliable method to assess its sharpness and performance. This technique involves gently gliding the blade across the nail, which provides immediate feedback about the razor's edge. If the blade grabs or cuts into the nail, it indicates that the edge is adequately sharp for shaving. Additionally, this approach allows the barber to check for smoothness and control, both critical for achieving an effective and safe shave.

Using a paper towel is less effective because it won't provide the necessary feedback regarding the blade's sharpness and grip. Similarly, shaving a small section of hair may not yield accurate results, as hair can vary in thickness and texture, potentially misleading the barber regarding the razor's edge quality. Testing on the skin of the arm is not ideal either; while it can gauge the razor's effectiveness, it poses risks of cuts or irritation, which a barber would want to avoid before performing a shave on a client. Thus, the method of testing on a moistened thumbnail stands out as the safest and most informative practice.