Two of my favorite tricks for checking to see if things a re square (i,e, a true 90 degrees – or perpendicular)  are the 3-4-5 triangel and the measesure the diagonals.

If checking to see if your deck is sruare to a wall or if the walls in your room are square  measure 3 feet from the corner along one wall horizontally make a mark. now from the same corner measure 4 feet from the corner along one wall horizontally make another  mark.  The distance between the 2 marks should be exactly 5 feet if the walls are square.

The math: Pythagorean Theorem: The sum of the squares of the opostirte legs of a right triangle are equal to the hypotinus squared.

 

When checking a rectangle or square box  to see if its square measure the diagonals they should be equal.  If not you are out of square.

Checking for square