r34-7Good question. I use a whetstone and free hand all my knives. Probably not the best way. But it is pretty easy to tell the angle when you place the bezel against the stone. Then you can tell if you are scrapping too hard / at too much angle if you feel it catching
There are bezel angle checking devices
erickongThat's a good one. A lot of tools are on the market to do this, with different names, but many end up being so big they can only take the grind angle of the knife blade itself, not the hone angle of the cutting edge that we try to match when sharpening the knife. It's the smaller ones like this that end up being useful.