How can I test if my hard disk is going bad?

If there are several I/O errors in /var/log/messages or one simply suspects the hard disks may be failing, smartctl can be a helpful tool in checking them. Hard disks can fail unexpectedly and it is always best to keep recent backups of all important data. Please keep in mind that even if a current or oncoming failure is detected, there may not be enough time to backu...