If your axes are not perpendicular to each other you will have a parallelogram on your extreme points.
Put a sharp tipped engraving bit (or sharp nail :)) in your collet and mark 4 dots on your table (use masking tape) at coordinates: 0,0; 0,MaxX; MaxY,MaxX and 0,MaxX, then measure DIAGONALS!!
If there is a difference, you are out of squareness.