How to measure an oil stone's grit?

Hi,
How can I measure a natural oil stone's grit? Thanks.

By comparing its scratch marks with hones of known grit.

Yes quite tricky to do unless you know someone in industry with a profilometer

It´s almost impossible to determine a stones grit. Any stones grit.

The scratch pattern can be deceptive. My Nakayama Nashiji sometimes leaves scratches that would not indicate its true finenes.

Basically you have to do some trial and error.
Sharpen a blade as fine as you can (without pastes) and hit your oil stone.
If the edge degrades, the stone is coarser than your finest stone.
Hit your next coarse stone (maybe a 6.000) and compare these two edges.
And so on.

This way you will not determine the grit of the stone, but its place in your progression