How to tell when drive has failed in RAID1?

[TNGL]

So as the title says, how do you tell when a drive has failed in a RAID1 configuration? Say if you have three drives... one fails, but the system runs as normal, how would you tell:
A.) If a drive has failed?
B.) What drive has failed?

This has got my head spinning, and I just need some clarification

 

[TNGL]

Thanks for the replies guys.
Do you think the Intel Matrix Storage Manager would tell me?

 

[TNGL]

Okay, thanks for the confirmation guys.