There are lots of 'theories' out there about how to answer this common question that involve burrs and arm hair but let's not make this personal here, ok? :-)
I'm a big fan of objective truth that I can demonstrate to everyone everywhere all the time. Call me weird, alright?
GHS has always needed an 'objective' testing method internally so that we could know if we were putting out 'quality' knife sharpening or not.
Unfortunately, you can't measure this kind of stuff with a dipstick so it took us many years to find the 'right' material to test our 'post-sharpened' knives with.
The big 'secret'? Common, ordinary butcher paper; really.
For us, slicing through this paper is to feel with our hands the level of resistance that the newly sharpened knife produces as it slices through.
Lots of resistance in the paper is bad, but little resistance is good and sharp to you individually.
For fun, you could try ordinary copier paper but the modern versions are so super thin that you'd have to apply steady 'tension' between your fingers across the paper itself to make it work for you. Don't cut your thumb off, ok?
Finally, this is our best GHS 'quality control' method that has been proven with many years of virtually zero complaints out in the real world.
What happens if 'they' discontinue making butcher paper in the near future? Back to the drawing board obviously. Sigh.