Hey Woody,
thanks for your contribution. This is a variation of the possibilities to check for rectangularity after adjustment by letting the machine mill something, but can also be part of a method of adjustment itself. It should be clear that in the end the relevant thing is always what the machine mills, thank you for pointing that out here, in the end you’ll have to mill points that you can check for rectangularity. This has been covered here on a few occasions, but it’s a bit scattered around the forum, e.g. here, here and here, or here Tom @TMToronto used a needle as a test tip in a self-made CNC pen holder and stitched into cardboard on the wasteboard in order to be able to check rectangularity.
The problem with this topic is the same as with many other topics, you need to read a while to collect all the information to finally be able to judge on what is the best practicable solution.
Today, I would suggest: Create a rectangle as large as the workarea as g-code and run it on the machine using a v-bit (with minimal cutting depth), and then use a bar gauge to check whether or not the diagonals are equal.