Lay a level across the top of the surface that you are measuring. Then, lay the level across the bottom. Look to see if both the sides are level, if not, the surface can't possibly be square. Make minor adjustments to the top and bottom until they are both level.
Take your tape measure and carefully measure vertically 3 feet from the top of one of the sides. Mark the measurement with a pencil.
Measure out four feet horizontally with your tape measure from the same corner, across the top of the surface. Draw a mark using your pencil at 4 feet.
Line up the end of your tape measure with the first mark and stretch it at a diagonal until it reaches the second mark. Look and see if the measurement reads5 feet or very close to it. Use the Pythagorean Theorem to determine if your corner is a true square.
Repeat this procedure on all corners. Make adjustments until all corners equal 90 degrees.