Why do people assume RIMS are that perfect?
I've seen 1mm variation from "bead" to bead" on cheap, NEW rims. Often with most the variation off to one side or the other.
You can often get an idea by simply taking an adjustable wrench to use as a go/no go gauge.
Adjust the jaws and slide around the rim.
You'll find high & low spots. Often enough to make dial indicators a waste of time.