As a part of your suspension that takes a lot of abuse, a good set of shock absorbers will dictate the handling and ride of your car. In conjunction with coil springs, the shock absorber exists to dampen the bouncing motion of the coil spring. When a shock absorber fails, it will show signs of leakage in most cases, signalling that the gas/hydraulic interior seals have broken. To tell if your car needs shocks, walk to each corner and give it a strong push downward. The movement should not bounce up and down. If it bounces more than twice, the shock absorber is no longer dampening the movement of the spring. Typically, shocks last around 100k miles, but in some cases they can last far longer.