Using a pencil line to check the accuracy of a square proves reasonably accurate. But I found that painter's tape provides a visual contrast for precise, easy-to-see results.
Painter's Tape Shows Precise Square

Apply a strip of tape pretty perpendicular—it doesn't have to be perfect—to a straight edge of the workpiece. With the head of the square tight against the workpiece and one edge of the blade over the tape, run a sharp knife along the blade, slicing through the tape, and remove the waste. Flip the square over and hold it against the knife line. If there's a gap, the square needs adjustment.

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