Printed a jig to test how parallel my blade was to the miter slot (turns out I didn't want to know :-). This attaches to a Microjig ZEROPLAY miter bar, so should be adaptable to any table saw which can use that miter bar. I've also included the OpenSCAD files for customizing distance to blade, height of dial indicator, etc. You could use the same jig to check the fence is parallel to the miter slots.

The main trouble with this was it showed how horrible my alignment is, so now I need to dig into the depths of the saw and try to make it better. More details at: https://tomhorsley.com/hardware/corexy/gallery-trunion.html