Identifying diskspace leakage

I am trying to identify tables with significant diskspace "leakage" due to
in appropriately low max_fsm_pages settings. I can see the results of
VACUUM ANALYZE VERBOSE output counts of tuples and unused tuples, and
understand that (1 - (tuples/unused)) is the amount of diskspace available
to be reclaimed with a VACUUM FULL or dump/reload.

Is there a way to identify the numbers of unused tuples without performing a
VACUUM? Is it stored in a system table anywhere? Other ideas on how to
identify disk bloat short of forcing downtime?

TIA.