Hey eaststreetcarry,
Please check Y too. You want to be sure that the assumption of the machine being out of square is right.
E. g. your workpiece, has it a right angle (or could you mill a workpiece which has one, a 90° corner), and then could you hold a joiner’s square to it? Is it square or parallelogram then?
One possibility would be to measure the distance of the outer corners of the feet, like this (imagine these little raspis were the machine’s feet):
Shit. You loose.