Roads marked with potholes, large cracks and sharp ridges that run across the pavement will wear out shocks faster. Frequently carrying heavy loads or driving on unpaved roads with deep holes or imbedded large rocks will do the same. Winter weather and road salt can also shorten shock absorber life by contributing to corrosion.
Having the shocks and suspension inspected at 40,000 or 50,000 miles, then annually after that, is a good idea. A thorough inspection should uncover what you actually need, if anything.
Longer braking distances or abrupt reactions through the steering wheel can also be caused by worn shocks, though your first inclination might be to blame something else for those problems.
If you’re having none of those issues, the shocks might still need attention. Bushings — the rubber and metal “cushions” at mounting points and connections — may be worn and allowing abnormal suspension movement or vibrations that can cause excessive tyre wear or put more stress on other suspension components.