Just found this info online.
Reading square with a pinch rod
It's vital to square up a case or opening immediately after assembly--before the glue dries. One way to check for square is to read the diagonal measurements from outside corner to outside corner with a tape measure. When the two measurements are equal, the opening is square. But clamps often get in the way, it's practically impossible to get a reading on the back of the case, and reading the outside corners won't tell you whether the inside of a deep case is square. A more accurate method is to use a pinch rod.
A traditional pinch rod is simply two sticks, sharpened at one end, that you pinch, or hold together, in the center. The modified version shown at right adds clamping heads that make things a little easier and more precise. Set the rod to the length of one of the diagonals; then check the opposite diagonal inside the case. Push the sticks into the case to read the entire depth. Keep adjusting the rod (and the case) until the rod fits equally between both diagonals.
