It will be hard to detect a “failing” RAID array. Your best option would be to go to the manufacturer of the RAiD or the manufacturer of your server to see if they would provide a “smart array intelligence” to give you that low level capabilities. Sometimes signs would be a performance decrease (I/O related) or occassional “hang ups”.
Normally hot spares are allowed to be added when the array was created only but you can attempt to review your manufacturer’s guide. With the exception of RAID 0 (basically no raid at all), you should be able to survive one failed drive. If multiple drives fail, then you would have issues with recovery from last known backup.
I would install a backup agent on this server and back it up often if you suspect something. Consider building a new server as a standby server. May consider utilzing XoSoft or Doubletake to replicate the changes between the two servers.
The best way to know the signs of the RAID array is to test it using some storage devices. You can also test other devices to determine if it is still working or not.