How to determine if your quilt is loaded squarely... the wrong way

This is what I have been working on for a very long time.  Four thread colors, lots of indecision, lots of learning (I have never done curved cross hatch), lots of ruler work, bright contrasting thread on black fabric.  So I get this crazy idea to attach the clamps (that I bought a year ago and have been too chicken to try) so that I can create some straight lines edge to edge in the outside sashing.  I was thrilled as I was going along until I got to the last third.  As you can see, I learned that my quilt was not square.  At least it was not loaded squarely.  Ugh!   More lessons learned.