No doubt you know about measuring the diagonals for square.
For flat, stretch thin wires tightly acroos the frame on the diagonal.
Make sure all the ends are firmly in contact with the surface of the metal not held up by a bead of weld.
With everything pulled very tight see if the two wires touch at the center; if they do not touch the frame is twisted with the corners for the top wire higher than the others.
If they do touch reverse their position so what was the top one is now the bottom and again see if they touch in the middle.
If they touch with either on top your frame is flat to withing the diameter of the wire.