Testing Spare Drives in Software RAID

[Hal Vaughan <hal@thresholddigital.com>]

I know mdadm offers a "--test" command, but it seems quite useless:

-t, --test
              Generate a TestMessage alert for every array found at 
startup.  This alert gets mailed and passed to the alert program.  This 
can be  used for testing that alert message to get through 
successfully.

That helps with knowing that alerts are working, but I'd like to be able 
to test the spare drive in an array.  Is there any way to test a spare 
device in a RAID to make sure it's in good shape and not likely to fail 
once it's needed?

Thanks!

Hal