I think the traditional thumb test is lightly dragging your thumb at right angles to the blade (not parallel or you'll get cut). A sharp edge will kind of catch as your thumb goes across it.

I don't depend on this particular test much. What I do is angle the blade so the light is reflecting off the bevel, then increase the angle so that it's just stopped reflecting, then strop it with my thumb watch what happens to the reflection on the fin. If you've got a clean bevel and the angles right and you have a good light source you can see the hairline reflection of the fin as it turns, and can tell from the angle where the fin reflects versus the angle where the bevel reflects whether (and where) the fin is forming and if it has turned into a wire edge. You need a good approximation of a point light source to do this. Fluroescent doesn't work at all; a clear incandescent bulb works if it's a 4-5 feet away from the razor; and the sun works great. I use clear incandescent.