There's no mention of these six drives being part of a RAID array or just a bunch of disks. If the former, I hope the array is RAID6 or RAID-Z2.
Assuming you're not booting the PC from any of the six hard disks, it shouldn't take more than an hour to check all drives individually, shutting down the machine every time you connect/disconnect a drive. If you disconnect a working drive, it will disappear from the operating system.
If you have Hard Disk Sentinel Pro (or similar) installed, you can identify the unique serial number of each drive together with its status (good or bad), then check the drive labels for the serial numbers (assuming you can read them without pulling the drives from the chassis).