Often it's a matter of feel, but I have noticed that when the water is very thinned down, almost dry, but runs up over the blade as i move it over the stone, that's a pretty good sign, assuming you've got the blade in good contact with the stone. For it to hug the stone so closely that the water has to run up and over, that bevel needs to have become pretty fine. This doesn't apply to the time when there is a good bit of water on the stone, but toward the time you might spray a bit more on. I also note if there are sections of the blade where the water doesn't flow up and over, often near the heel.