Spring problems are generally easy to identify. A shock absorbs energy by forcing oil through valves whenever it is moved. It takes a lot of energy to push oil through the valves so when the spring is done pumping, it doesn't have much energy left to keep bouncing. Imagine running 50 yards on bare ground - you could do it and have plenty of energy to run back again. Now imagine running the same 50 yards through 6 inches of mud.
Shocks also control the reaction of the body to road undulations.
A stiffer shock tends to transmit more road irregularities to the driver but will also not pitch and roll as much as a vehicle with softer shocks. Thus shocks, like springs, can be changed to obtain a personalized ride. Shock absorber condition is difficult to evaluate. Shocks are wear items, like air filters or tires, and do need to be replaced occasionally. The Car Care Council recommends inspection at 25,000 miles and every 6,000 miles thereafter.