The only proof that a knife has been sharpened is a burr: a fine curl of metal formed on the side opposite the one worked on the stone, hard to see but perceptible with a fingertip. If the burr forms continuously from heel to tip, that side is finished. If you turn the knife before it forms, it has not been sharpened, only polished, and within a few days it will stop cutting again.
Sharpening means removing metal from the edge; it is not polishing. As the stone abrades one side, some of the removed metal rolls over the edge to the other side and leaves a fine curl there. This curl is called a burr, and it proves that the two sides have actually met, because metal can roll to the opposite side only when the edge has been ground down to zero. So a burr is not a flaw; it is the sign you are looking for. The goal is to form it first, then remove it cleanly. A knife lifted from the stone before a burr forms still has its cutting edge at the old worn spot, and the first onion will show that.
Do not look for a burr; feel for it, and the direction of the check is crucial. Draw your fingertip perpendicular to the edge, away from the edge; never slide it along the edge, because a finger moving along a sharp knife will be cut immediately. If you feel a rough line on the side opposite the one being worked, a burr has formed. If you feel nothing, the metal has not yet reached the edge, so continue on the stone. Check with a clean, dry finger; a wet or oily finger will not feel a fine burr, and you may think the job is done.
The most common mistake is checking only part of the burr. The middle of the knife is easy to sharpen, but the heel and tip are left behind because they do not sit fully on the stone; if there is a burr in the middle but not at the tip, the job is not finished. So check at several points from heel to tip and work on any missed area until a burr forms there. A knife that has not been sharpened at the tip will cut a tomato but crush garlic. For the heel and tip, change the knife’s path on the stone, not its angle: lift the handle slightly while working on the tip so the curved section lies flat on the stone.
After a burr forms, turn the knife over and repeat the same process on the other side; this time the burr rolls onto the first side. Once the two sides are done, the burr is no longer wanted and should be removed on the stone’s fine side with progressively lighter passes, alternating between the two sides. Hold the knife at a steady 15–20° angle to the stone; if the angle changes with each pass, the edge rounds over and the burr will not come off cleanly.
The final check is done by cutting, not by checking for a burr. An edge with the burr removed should slice a taut, thin sheet of paper without snagging or changing sound; it should enter the skin of a ripe tomato without pressure. If these two tests do not pass, burr fragments remain on the edge, and they will break off during the first use, making the knife seem to have gone dull within one day. A sharpening stone is needed only occasionally; once every 2–3 months is enough. A honing steel does not sharpen; it only straightens a bent edge.