Use one of these.
They have a magnetic base so it gets stuck to a reference datum point that won't move. Set the arms so the dial gauge pointer is on the rotor and slowly turn the wheel. If the rotor is true and straight then the needle won't move, if its bent then the needle will swing back and forth.
I've got one if you wanna come for a ride out to Pukekohe some time, also got a set of bike stands that lift the front by the bottom steering head mount.
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