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

You can check hard disk errors using utility for SMART disks under Linux / UNIX like operating systems.
Step 1. Enable SMART:
# smartctl -s on -d ata /dev/sdb
Step 2. Run self-assessment test for hard disk:
# smartctl -d ata -H /dev/sdb
More detailed test:
# smartctl --attributes --log=selftest /dev/sda
You may get even more details with
# smartctl -d ata -a /dev/sdb

Sample Failing Hard Disk Detailed Report:
# smartctl -a /dev/sda