Naturally, I found the answer right after posting this thread!
- volume reallocation measure -vserver [SVM] -path [volume path] -once true
That produces a job ID, which you can monitor with:
- job show -id [job ID] -fields starttime,endtime,state,progress
When the job completes, the result is shown in System Manager -> Events. Filter the 'Event' column on 'wafl.reallocate.check'. A result of 1 is optimal. Higher numbers are bad. The event will advise if reallocation would be worthwhile.
Example: "wafl.reallocate.check.highAdvise: Allocation check on '/vol/mail_archive' is 13, hotspot 0 (threshold 4), consider running reallocate"