How do I measure the error of my ohmmeter
They calculate it by knowing the tolerances of all the components they used, and knowing how much each component influences the overall measurement. Then they pad that with a fudge factor to account for how the component values will change as they age between calibrations.

It isn't a simple proposition, and sometimes it's more expedient to do a Monte-Carlo simulation of the circuit that randomizes the component values and do a statistical measurement of the results.