I use the thumb test. I have never been able to get the HHT to work for me with any razor honed by anyone, even the best.

You're supposed to use hairs as close as possible to the ones you're actually cutting. Otherwise I wonder how reliable it will be.

There's a visual test that someone mention here where you look head on at the edge in a strong light and see if you can get the edge to reflect light at any spot (that's a sign of dullness). This is an old knife sharpening test. The problem I have with it is that a razor needs to go past that point. You will pass that test before the razor is keen. In fact, you'll probably pass it when the razor comes off the 4K.

The ultimate test is the shave. The only reason you need these other tests is that you don't want to have to jump back and forth constantly between honing and shaving. So, a useful sharpness test is something that has a high probability of telling you when you'll pass the shaving test.