Postgres - how to check how much OS space can be recovered

I have a huge tables where a lot of DELETEs are being made. From time to time Im vacuuming tables to restore some space back to the system, by executing below

  VACUUM FULL VERBOSE ANALYSE table_name

I know autovacuum should do the job, but I cant see any space usage reduction over the time. Full Vacuum it is apparently quite long process and sometimes I realize it wasnt worth effort, because I recoverd only fre Gigs. Other times I can get dozen of space

So I wonder if there is any way to check in Postgres how much space can be released back to OS before any operation are taken / which tables need vacuuming

Im using version 12.

**** Edit ****

In short I need rough estimate of wasted space I can get back after vacuuming.

I'm not sure about "released back to OS".

But for rough estimate of wasted space I would do:

SELECT 
        schemaname , relname , 
        pg_total_relation_size(relid) AS total_size,
        n_live_tup , n_dead_tup , 
        n_dead_tup * 100.0 / 
        CASE n_live_tup WHEN 0 THEN 1 ELSE n_live_tup END  AS dead_ratio,
        pg_total_relation_size(relid)::numeric * n_dead_tup  / CASE n_live_tup WHEN 0 THEN 1 ELSE n_live_tup END AS wasted_space
    FROM 
        pg_catalog.pg_stat_all_tables 
    WHERE
        schemaname = 'public'
    ORDER BY 
        wasted_space DESC;

Logic:

• get ratio of dead tuples
• multiply on relation size

Of course it's a very rough estimation, real results will vary.