You can also use a diagnostic tool to check the health of the hard drive. It can give you a warning when certain attributes are beyond specifications. The warnings generally do not mean a hard drive failure is imminent, but that it can eventually fail if ignored for a prolonged period of time.
I use a program call Defraggler. It's actually a pretty good piece of software to defrag the hard drive and it also has a "Health" tab where you can check the status of your hard drives. I love multi-functional programs.