Once the perimeter boards are in place you can “square” the deck using 2 tape measurers. This will only work if the form you are testing has symmetrical geometry. In the case of a square or a rectangle you will measure diagonally from corner to corner on both sides. These measurements should be equal. In the case of an octagon you will need to select two distances from opposite sides of the polygon.
As the joists are added they should not only add strength and rigidity but should also in effect square your deck if they are cut to the appropriate lengths. Obviously the more square the frame the better. Slight tolerances can be adjusted before the joists are installed to prevent exaggeration. Compare the opposite lengths of the perimeter of squares and rectangle forms; these lengths should be the same. Using a square and a 4’ level will also help verify that your deck form is level. You can also use the 3-4-5 rule to verify right angles.