I would like to add my two bits to the wiki blurb's section on the hanging hair test (HHT.) Even if the razor doesn't cut the hair, the HHT still can give good feedback on your edge. When performing the HHT, drag the hair over the blade edge and watch the tip of the hair. When a razor is dull, the end makes little motion. As the edge becomes thinner and thinner, the end of the hair vibrates more. Finally, it will become sharp enough to cut the hair.

But, again, the vibration before it starts to cut is related to the sharpness of the blade, and can give you good feedback.

And, remember, with the HHT everything anyone says might or might not apply to your hair and technique. However, the above should apply in most instances.