Reply by Dallas

Harbor freight sells a set of three aluminum squares. The two largest are adjustable so to make them square, put them on a known square surface, scribe a line, then flip them over to check that the marks are exactly the same at 180°. If not, adjust and check again and again until they are square.

http://www.harborfreight.com/3-piece-l-square-set-with-levels-98556.html

The problem with buying expensive squares that are square from the factory you may ask?..... The first time the bugger hits the floor it is no longer square so you’ve wasted your budget.

-- Improvise.... Adapt...... Overcome!