Now i am going to be ridiculed for this statement, but i have tried them all except mechanical testers e.g. bess machine, and paper has one major flaw. I have done many tests using a micro-scope which takes you into a realm you have never been to unless you have tried using one. And the flaw is, you can have a perfectly apexed edge, deburred and sharp and crisp. But when cutting paper it can still catch making you believe that you still have a burr, but it is remnants of burr or even length of wire edge sticking to the bevel, so the edge is fine, but the paper will catch. So for most of us who use knives as tools, this doesn't matter because you still have a very sharp working edge. But if you can slice foam packing pieces into fine slices, and the blade has to slice through easily, you have a sharp knife. If the blade struggles or doesn't slice through with ease you have to go back to the stone. With practice when you get it right you will know straight away, and when you get it right that is your benchmark, after that you will know when to go back to the stone.
