The quality of the ride is probably the first indication of rear sub frame bushings that needs to be replaced although the different suspension links can also be culprits. With suspension links it is easy to manually test for loose bushing. When there are no loose bushings with the suspension links and the ride is still mushy (like the rear end is moving sideways) then it is likely that the rear sub frame bushings needs replacing. The appearance of the bushing will also confirm the need for replacement. There are two front and two rear bushings on this sub frame. In most cases the front bushings only will need to be replaced. After replacement of these bushing and assuming that the various suspension links ( 3 per side) are solid, the ride should be similar to a train on a railroad track, i.e. same as when the car was new. The front end suspension might also need repairs: loose tie rods, upper and lower arms bushings, ball joints and surprisingly enough: the steering damper (will affect stability). After redoing the front and rear control joints, the car should be ok for another 100,00 miles.
Hope this is helpfull.
Regards,
jacquesrober.

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J.R.