IMHO, 3 methods I use to check sharpness. This one I've used the longest and it works fine, take a piece of pine and put it your vise and see if you can shave the end grain of the scrap. It should go with little effort and most important, it should slice the end grain not tearing it. Oddly the second method I had discounted for a long time, until I found it seems to be a degree more accruate. Simply place the bevel gently and with it's own weight against you thumb nail and try to drag the face of the bevel lightly. It it's stuck in place you're sharp, if you can drag the bevel, it's starting to get dull. Another method that I use is to look at the edge under a light and magnifier if I see the end of the bevel, it's not sharp.
Knowing you're sharp is only part of the battle, getting sharp involves consistant angle, and doing it free hand takes practice, not to "roll" the edge, but stay close enough to make the edge (Initiateing the cone of silence don't tell anyone here, but I also use a jig****cone now off***). Some here have mentioned the wire edge. If you don't get that edge, you have not "zeroed" the angle on the bevel or you've rolled the bevel and taken the wire edge with it and thus the sharpness. You can make a mirror all day long, but if you don't take that metal completely off the end of the bevel, your not sharp. I havn't read all the posts and answers closely, are you glueing the sand paper down? I have had wetted down paper create a kind of wave in front of an edge and it dulls the results. Others may have luck with not glueing, I don't. For that reason I find it easier to use stones.