I use two 4 foot pieces of extruded aluminum, very straight 80/20, across the mill bed, then use a 12” speed square on top of them, and push the back of the square against the backstops to confirm the backstop alignment. It should be done at least two times per backstop, once at a full 90° and also partially laid down.
FWIW, my backstops are set about 1/16” or so inboard so they will straighten up with clamping pressure.