A Knife Expert on How to Really Sharpen a Blade - Reveal the edge. To check the sharpness of your blade, turn the knife edge up under a strong light, and look at it carefully. A sharp edge will look like a thin black line. Any reflection spells trouble. Dull spots will shine. Minute nicks and burrs will show up as tiny glistening pricks of light. If you see them, head back to the stone.