put the blade to the paper test
Even the best kitchen knives will dull over time with regular use. To determine if your kitchen knife needs sharpening, put it to the paper test. Hold a folded, but not creased, sheet of newspaper by one end. (You can also use a single sheet of basic printer/copy paper.) Lay the blade against the top edge at an angle and slice outward. If the knife fails to slice cleanly, try steeling it (see below). If it still fails, it needs sharpening.
In our kitchen knife sharpener research, we examined blades—from sharp (smooth and sharp) to dull (all chewed up)—under a high-powered laboratory microscope at the Massachusetts Institute of Technology’s Department of Materials Science and Engineering.