How to identify failed drive in RAID

Hi, I have setup a RAID on my MacPro with the four drive slots. drives 1 and 2 are stripped (RAID 0), 3 and 4 are stripped. then I mirrored 1+2 with 3+4. (not sure if this a RAID 1+0 or 0+1 that is why I have described it). When my machine failed to boot and launched recovery mode to find out that the mirror (3+4) was degraded. How do I identify exactly which drive has failed (i cannot hear or see any physical defects)? Is it just a case of replacing the drive and clicking rebuild or is it more difficult than that?

Is there any instructions on this? If complicated step by step instructions would be appreciated. thanks