There is a free utility being developed from Apple open source code that provides, for those external FireWire and USB drives that support what the developer calls "SCSI ATA Translation" in the drive enclosure, the ability to run SMART tests.
Version 0.5 is at MacUpdate. The developer has posted 0.6 at his own site:https://github.com/kasbert/OS-X-SAT-SMART-Driver
To download version 0.6, click the link named:
then click View Raw.
The installer is in a disk image. Be sure to read the Read Me, which mentions that the external drives must be unmounted before the installer is run, and which provides two Terminal commands that remove the files, after which a restart must be done.
I do not have any information about this utility beyond what is in the Read Me, and none of the drives I own have suitable enclosures, but it is encouraging to see that in at least some cases, SMART testing can be done on external FireWire and USB drives.