Get a straightedge (like a saw guide), ruler and a calculator.
Make a 3:4:5 triangle or a 5:12:13 triangle as precise as you want, within 1/32" or 1/16" or some other. But don't get scrambled up on converting inch fractions to inch decimals, or just use the metric scale on some tapes.
Both sides squared for the right angle have to add to the last side squared, 3^2 + 4^2 = 5^2.
And you can use this method to make your own square from scrap strips and three bolts. If one is an angle strip it'll rest against the drywall edge.
With your tape and calculator you can also check the store-bought Chinese-made square before you buy. It's probably good to within 1/64".