The usually signs are:
1. File corruption
2. Errors when read/writing data to disk
3. Disk fails to boot
4. Unexpected noise from drive.
5. Disk not recognised at bootup.
If a drive is more than 5 years old then it may be time for a replacement. Its not unusual to get 1 or 2 bad blocks on a disk and some utilities can map them out. I believe HDToolbox does have this facility to mark blocks as bad and save it in the RDB area.
