software raid - How to Detect Hard Disk Failure

I have a software RAID 1 setup in my CentOS 6.2 and set to be bootable in any of the HDDs in case one of them fails.

Questions:

1. How can I recognize if one of the HDDs fail? or early signs of failure on CentOS? (preventive maintenance)


2. If one of the disks fails, what are the things to be done? Like any DATA RECOVERY methods. (corrective maintenance) or (how to copy the data in the remaining HDD and copy it to the new HDD)

I would gladly appreciate if you could give me any references.

Update:

I tried to boot in only one of the disks. I removed sdb first and the system successfully booted in sda. Then I removed sda and booted in sdb and still successfully booted. But when I put them back together and executed cat /proc/mdstat & mdadm -D /dev/md0, it shows that one of the disks is still removed.