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.
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.