Measure from the end on both edges and use a straight edge to join the two marks. Unless you have drywall square. https://www.homedepot.ca/en/home/p.48-in-drywall-t-square.1000811812.html

Definitely a drywall square. Works well on all 4x panels. Instead of a speed square, I'd use a framing square and Neal's method if I didn't have a drywall square.

You can also use either the framing square or the drywall square to check the end cuts of the panel to verify those factory cuts are square.

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".