Flatness across the bed? Measure gap between tip of tool and bed at various points.

Squareness of gantry? Drill four holes at corners of a square, fix pins in, and measure diagonals. I use a box of, say, 4mm drills for this - drill holes with one drill and jam four others in holes by their shanks. Thick bit of MDF works well for this.

Check that tool returns to same position after move - useful check for backlash.

Do, say, 100 there-and-back moves in each axis and check that tool returns to same position. If it slowly drifts, this might indicate an issue with the polarity of the "step" signal on that axis - not an unknown but sometimes difficult to spot issue, easily fixed.