You make a box straight edge.

First, you make a surface gauge. A heavy weighted base with a pointer you can vary in height and distance from the base. You scrape/grind the base flat using a surface plate as reference.

You take something you think is roughly square (a long rectangular prism is good), scrape/grind one face flat with one of your surface plates. We'll call this face 1. Then scrape/grind the opposite face flat, and using the surface gauge to check height (position the box with face 1 down on a surface plate. Position the gauge next to it, with the point touching face 2 somewhere. Slide the gauge back and forth: if it gets pushed up face 2 is tilted up in that direction with respect to face 1. If a gap forms under the tip, it's tilted down.) and thereby get it parallel with face 1.

Then scrape/grind two faces which are adjacent to each the first and opposite each other flat. You now have a shape like \_/ or |_/ or such, though with a top parallel to the bottom.

You stick your surface gauge along one side, touching at the base and at some point near the top of that side with the indicating point. You then turn the WIP straight edge box 180 degrees, and see if the other side touches the indicating tip or the base first. The distance from touching both at once tells you how far out of parallel that side is from the first. You then proceed to scrape them both to be a bit more flat and parallel. Then you turn the box onto its top (face 2) and repeat that. Using 180 degree rotations in two dimensions will get you a good square.

Another thing that can be used as a check (if you can draw) is to build a compass and a straightedge. Side 1 of your box is a good straightedge! You can then use basic geometric construction to make a right angle, and can compare any finished box to that angle.

Practically, the fastest and easiest way would be to turn a good cylinder and measure the diameter at several points (or roll on a surface plane) to make sure it's not tapered, because the face will sit square to the sides even if the axis cutting the face is concave.

More fundamentally though, you can use a similar method -- start with a surface plate, and make three almost-90-degree right angles. Label them A, B, and C.

Scrape A and B so they perfectly mate with each other while they sit flat on the surface plate. They might be something like 89 degrees and 91 degrees, so scrape C to be a copy of B, and then mate it with B. From that you can tell if they're both acute or obtuse, do the correction, and repeat.

Same method essentially. You need three roughly square pieces that are checked for bearing against each other repeatedly, but this time you also have intermediate steps of checking each face for flatness against the master.