The traditional thumb test to evaluate shave readiness is as follows:

Hold razor fully opened by the handle with point "pointing" towards the ceiling. Lightly place moistened thumb at heel of razor edge, first 2 fingers on spine, and lightly move the thumb, not the razor, move the thumb lightly along the edge towards the point. The thumb should "stick" or bind along the edge. This is because the razor is actually slightly cutting into the outter layer of your skin. When the skin binds, stop, lift thumb and replace higher up the edge and repeat until you cover the entire edge of the razor. If the thumb glides smoothly along the edge without a "sticky" feel, the edge is not cutting into the skin and is not yet sharp enough to shave.

I can't emphasize enough that this test is done with a light touch, or you may do more cutting than intended. The fingers on the spine help control pressure and movement of the thumb.

Scott