I've just inherited a vintage British 25 mm micrometer, and it's precious to me.
You can test a spirit level by reversing it, and similarly you can for an engineer's square. We can test a right angle via the 3-4-5 formula. Absolute verticality can be proved with a rock on a string. And NASA can detect the constituents of exoplanet atmospheres thousands of miles away just by peeking at them. Is there some mathematical 'trick' that we can perform to test the accuracy of a micrometer?
I'm quite prepared to accept no.
This question excludes expensive gauge blocks.