Reclaiming unused indexes disk space
Perhaps you could start with:

SELECT *
    FROM pg_stat_all_indexes
    WHERE schemaname NOT IN ( 'pg_catalog', 'pg_toast' )
        AND idx_scan = 0
        AND idx_tup_read = 0
        AND idx_tup_fetch = 0
        AND indexrelid NOT IN (
            SELECT indexrelid
                FROM pg_index
                WHERE indisunique = true
                    OR indisprimary = true ) ;


No guarantee that the indexes that pop up aren't ever used, but it should provide a list to start looking at.