Here’s how I test jnats. If you’re just testing one, it’s easy, you can just hone up a razor with mellow diamond plate slurry and shave with it. If you have two identical razors - I use Gold Dollars or Gold Monkeys as test razors - you can hone one on a known good hone/tomo and another on the stone to be tested and compare the shaves.
If you’re comparing two or more stones, as in grading a set of Mikawa nagura (many used ones are not stamped at all), then having a piece of polished steel is helpful. You can use a knife or cleaver provided the steel has been polished to a high finish with all of the factory finish removed and no scratches are remaining - that last part about scratches is important. The Masahiro gyuto in the image was a project knife for thinning and polishing, but it provides a large area of polished steel. You will need a loupe of your choosing. I like the B&L 7x Hastings Triplet, but if you want a cheaper one the illuminated Chinese 20-30x (they’re all about 10-12x, lol) work fine except near the edges of the field of view.
The razor is a badly worn near wedge Japanese that was sent to me as a gift when I bought another razor from a friend. I thought, ‘What am I going to do with this thing?’. As it turns out, I use it more than the razor that I bought, but for testing! The bevels are about 1/8” or 3mm wide, and the spine wear is almost 3/16” or 5mm wide, plenty of space to look at with a loupe.
In grading the Mikawas, which are very good BTW, I did one side of the razor with the softest one and the other side of the razor with the next hardest one and observed the scratch pattern, also looking for any rogue scratches. I labeled them with removable labels. Next I tried the hardest one on the side opposite the next to hardest and compared scratch patterns and labeled it. Next up, to confirm the grading, I honed up three razors on a very hard stone using the slurry from each and then shaved with them. It doesn’t take a lot of this to determine the sequence of coarsest to finest.
Matching razor finishing tomo naguras to a razor finishing hone has to be done by shaving really, the scratches are so fine a loupe is not that helpful. A microscope is quite helpful here, but I don’t have one, and for the final edge, shaving is always the definitive test regardless of what you see under a microscope.
Lot’s of fun if you’re a honing nerd!