Another way to check for Square is using 3,4,5 method. measure from bottom right corner(looking from above) up 3' and mark, then from same corner measure left 4' and mark. now the distance from mark to mark should be 5' if you are square. a2xb2=c2. You can also use this for the walls or anything you want square. This method is easy to use if you are working by yourself and/or on big projects.