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No matter how you strop, whether by hand or with power, the surface you are working is always face down and out of sight. This makes it difficult to see if you are holding the tool at the correct angle.
I use a Sharpie® marker to draw a broad mark on the side of the blade to be stropped. Stropping removes the ink only from the parts of the blade in contact with the strop. If the ink is removed from the heel of the blade and not the whole bevel, you are holding the tool at too low an angle. If you hold the tool at too high an angle, the ink is removed only from the tip of the edge, which can produce a double bevel. Slightly adjust the way you hold the tool until the ink is removed evenly from the blade; a few additional strokes will polish the tool and remove any remaining ink.
I also use this method when sharpening my tools with a stone or grinder. This way, I can be sure I have only a single bevel on my tools.
Source: Woodcarving Illustrated