One way to check for lopsided or equal sag at the rear is to check the vertical clearance between the underside of the axle and the chassis cutaway where the rubber rebound stop is. Do it on flat level ground and note that with the car stationary the front suspension will significantly control roll attitude, so make sure the front suspension is at the same height both sides by checking the length of the rebound springs at static ride height.
OK this clearance varies between models and years, but if it is much more than around 30mm unladen then the chances are that the springs have sagged. You will get a good idea with the two finger in the gap trick, is that a rule of thumb? Regards PJB.
