As an aside for checking square, measure diagonals to make sure they are the same (watch out, a trapezoid is also checked this way).

To check a framing square (not a "t" square): Place the square along the edge of a work table which is a known straight line. Draw a line along the leg which is perpendicular to the edge against the table. Next, flip the square over and draw the same line with the square on the table edge pointing in the opposite direction. This will double any error and allow you to see any variations. Sorry about the complicated nature of this description.