Personally, I find that filespaces are a good indicator.  Generally, the
backup_end should be more recent than backup_start.  Backup_end should also
generally be within the past day or it missed.

select count(distinct(node_name)) from filespaces where backup_end <
(current_timestamp - (1 day))

Alex Paschal
Storage Administrator
Freightliner, LLC
(503) 745-6850 phone/vmail