[dsraven]

sorry, forgot to say the check for the rear axle being 90 degrees to the frame would assume the axle is also centered in the frame, side to side. otherwise those measurements from the axle flanges to a common center point would be skewed.
if you did the plumb bob thing with marks on the floor, drop a line down from each rear axle flange center. chalk a line on the floor to connect the frame center line marks from front to rear and measure out from that chalk line mark to each axle flange mark to ensure they are both the same dimension to prove the axle housing is in the middle. then do the diagonal to a common center point after that is established.
thats just my way, lots of other methods probably coming right up