If you really want to get down to fine-tuned levels of sharpness there are a few more tests you can use. My personal favorite is to take a piece of paper and hold it vertically. If you try to cut it with a dull knife, the paper will crumple beneath the knife. A sharp knife will cut it cleanly when use a slicing motion to cut through the paper. A razor sharp knife can cut the paper cleanly by just pressing down on the edge of paper without any slicing at all.