How to predict how much space a VACUUM FULL would reclaim? Is there a way to determine in advance how much disk space a VACUUM FULL on a particular table would return to the OS? Therefore you can decide whether it's worth the cost in doing so.
Bonus if there's a simple query to do this for every table in a database/server (rather than doing each one separately).
You need the pgstattuple extension to get the amount of free space.
So you could run
SELECT t.oid::regclass AS table_name,
       s.table_len AS size,
       dead_tuple_len + s.approx_free_space AS reclaimable
FROM pg_class AS t
   CROSS JOIN LATERAL pgstattuple_approx(t.oid) AS s
WHERE t.relkind = 'r'
ORDER BY (s.dead_tuple_len::float8 + s.approx_free_space::float8)
       / (s.table_len::float8 + 1.0) DESC;

where size and reclaimable are in bytes.
Be warned that this query is somewhat expensive.
That should give a pretty good estimate of the space that can be reclaimed, but in practice it will be slightly less, because

there is always some free space in each 8kB block that is too small to fit another row

some of the dead tuples might not be reclaimable if you have long running transactions