Push a diamond point into a hard surface for a specific time period and measure the indentation made. In short, that is the Knoop Hardness (KHN) test developed by F. Knoop (and others) in 1939 under the flag of the National Bureau of Standards.
A Knoop indenter was developed for this test – it is a specifically shaped diamond with very specific measurements (ratios). More details, including the formula can be found here.
Even more details, specific to steel hardness can be found here.
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