I've not been able to understand this feature from Google or KB searches.
- My Macintosh HD has no "S.M.A.R.T. Status" in Disk utility
- The HD does show as "S.M.A.R.T. status: Verified" in the System Information report
- My Thunderbolt-connected external drives do show "S.M.A.R.T. status: Verified" for both hard drives in Disk Utility.