Use the factory edge of a piece of plywood or an edge that you know is perfectly straight to align the short side of your square. Draw a line along the edge of the long side of the square. Then flip the tool over, aligning the base of the mark with the same edge of the square; draw another line.
If the two marks don’t align, your square is not square.
When buying a square, it’s a good idea to check its accuracy before leaving the store. Bring a pencil and a long piece of paper or cardstock with you when you shop. Grab a handful of squares and head over to a sheet of plywood. Follow the steps above, marking the paper not the plywood, and then purchase the square the passes the test.