This is a simple squareness reference made by using a 4" gauge block.
If you own a gauge block set you know the squareness of a block is not guaranteed. But for squaring up the axis of a 3d printer my blocks are way better than what is needed.
The holder simply secures the 4" gauge block to the linear guide rail bearing block. You then run a dial gauge up and down the 4" face by using the printer's Z axis. Adjusting the axis tilt until the reading is constant. Next, rotate the dial indicator 90 degrees and repeat. Ultimately you want a Z axis that is 90 degrees to the print bed.
Typical measurement indicator
Tramming which is adjusting the three motion axis to be right angles to each other on any machine takes longer than you expect and most printers are not even designed to be trammed so you will probably need to improvise.
Tramming is usually done in software however that causes distortions in the print that I want to avoid.