Well, of course many here will tell you to shave with it. If it shaves well, then the razor is sharp enough. Go no further.
But what if your shaving technique is not "there" yet? You might have a razor that is sharp enough, but still get a poor shave. So how do you tell then? Should you just assume that you need to re-hone, re-strop, and then re-test shave? That could be a lot of wasted time and effort.
After stropping do the "Thumb Pad Test." Of course this will also take some time to learn. Determining just what a proper keen razor edge feels like on the wet thumb pad is a skill in itself. It really helps to have an in person tutor.