Best indication is to setup a set travel path and measure it wouldn't it? I've put a vernier in my vice before, set the machine against it in a positive direction for a couple of mm to make sure it's correct. And then set it to run 100mm (or so) to see what is the actual.
Speed and increasing it will eventually cause missing steps but speed is not the way to measure accuracy. Also it might be to advantage to download a torque curve for your motor as it drops off with rpm
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