Visually inspect the shock absorber. If there is evidence of leakage and the shock absorber is covered with oil, the shock is defective and should be replaced.
If there is no sign of excessive leakage (a small amount of weeping is normal) bounce the car at one corner by pressing down on the fender or bumper and releasing. When you have the car bouncing as much as you can, release the fender or bumper. The car should stop bouncing after the first rebound. If the bouncing continues past the center point of the bounce more than once, the shock absorbers are worn and should be replaced.