Hey Bill,
one simple way to verify X/Y-squareness of the machine is taking a workpiece nearly as big as the workarea, fixing it at marked points to ensure repeatability, then program four holes that form an exact square to be milled in the outer corners of the workpiece, milling them, and then rotating the workpiece by 90° and repeat the program. Et voilà.
It simpler than the tiled cut example which also shows if the machine has un-squareness.
See also this for stitching instead of milling.