The ride quality of shock absorbers usually deteriorates gradually. Most people do not notice it in every day driving. If you suspect you need new shocks, visually inspect them for leakage: (a light coating of oil on the shock housing is normal) heavy build-up or leaking on the ground is not. Excessive pitching and/or rolling around corners or during braking is an other sign of bad shocks. If you have the shocks off the car, extend and compress the shock several times. Any lack of resistance in either direction of movement constitutes replacement of the shock. One misconception is that if the car is sagging, it needs new shocks. This is not necessarily the case, as shocks to not control ride height, the springs do.