I've got a huge work table with a square top, so it's easy for me
The end of the material will have a "short" and a "long" corner. The "long" corner sticks out farther, while the "short" corner is recessed.
Eyeball the end of the material and locate the "short" corner. If you can't tell by eye, the material probably isn't out of square far enough to matter.
Make sure your material is as flat as possible, and not stretched out.
From the "short" corner, measure along the length of the material 3', then across the width 4'. Make marks at both of these measurements, on the edges of the material.
Now measure between your marks. If this measurement is close (within 1/2" or so) to 5', you're square enough. If not, adjust the width mark (the 4' measurement) so you DO get 5' between the two marks. As you adjust the width mark, make sure it's still 4' from the "short" corner.
When you have 3' along the length edge, 4' across the width, and 5' in between, you've got a right triangle. From the "short" corner to your 4' (width) mark is now a straight line, square to the long edge of the material.
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