Shocks & Struts
You need new shocks and/or struts if your original shocks or struts are worn out, damaged, or leaking. Leaking is easy enough to see, just look for oil or wetness on the outside of the shock or strut. Damage is also apparent with a broken mount, badly dented housing, etc. Wear is often more of a subjective thing to judge.
There are also instances where the original equipment shocks may not be worn, damaged, or leaking, but may not be adequate for the job they're being asked to do. In such cases, upgrading the suspension with stronger, stiffer, or special shock or strut may be recommended to improve handling, trailer towing, hauling overloads, or other special uses.