How to tell which indexes are not being used

WITH table_scans as (
    SELECT relid,
        tables.idx_scan + tables.seq_scan as all_scans,
        ( tables.n_tup_ins + tables.n_tup_upd + tables.n_tup_del ) as writes,
                pg_relation_size(relid) as table_size
        FROM pg_stat_user_tables as tables
),
all_writes as (
    SELECT sum(writes) as total_writes
    FROM table_scans
),
indexes as (
    SELECT idx_stat.relid, idx_stat.indexrelid,
        idx_stat.schemaname, idx_stat.relname as tablename,
        idx_stat.indexrelname as indexname,
        idx_stat.idx_scan,
        pg_relation_size(idx_stat.indexrelid) as index_bytes,
        indexdef ~* 'USING btree' AS idx_is_btree
    FROM pg_stat_user_indexes as idx_stat
        JOIN pg_index
            USING (indexrelid)
        JOIN pg_indexes as indexes
            ON idx_stat.schemaname = indexes.schemaname
                AND idx_stat.relname = indexes.tablename
                AND idx_stat.indexrelname = indexes.indexname
    WHERE pg_index.indisunique = FALSE
),
index_ratios AS (
SELECT schemaname, tablename, indexname,
    idx_scan, all_scans,
    round(( CASE WHEN all_scans = 0 THEN 0.0::NUMERIC
        ELSE idx_scan::NUMERIC/all_scans * 100 END),2) as index_scan_pct,
    writes,
    round((CASE WHEN writes = 0 THEN idx_scan::NUMERIC ELSE idx_scan::NUMERIC/writes END),2)
        as scans_per_write,
    pg_size_pretty(index_bytes) as index_size,
    pg_size_pretty(table_size) as table_size,
    idx_is_btree, index_bytes
    FROM indexes
    JOIN table_scans
    USING (relid)
),
index_groups AS (
SELECT 'Never Used Indexes' as reason, *, 1 as grp
FROM index_ratios
WHERE
    idx_scan = 0
    and idx_is_btree
UNION ALL
SELECT 'Low Scans, High Writes' as reason, *, 2 as grp
FROM index_ratios
WHERE
    scans_per_write <= 1
    and index_scan_pct < 10
    and idx_scan > 0
    and writes > 100
    and idx_is_btree
UNION ALL
SELECT 'Seldom Used Large Indexes' as reason, *, 3 as grp
FROM index_ratios
WHERE
    index_scan_pct < 5
    and scans_per_write > 1
    and idx_scan > 0
    and idx_is_btree
    and index_bytes > 100000000
UNION ALL
SELECT 'High-Write Large Non-Btree' as reason, index_ratios.*, 4 as grp 
FROM index_ratios, all_writes
WHERE
    ( writes::NUMERIC / ( total_writes + 1 ) ) > 0.02
    AND NOT idx_is_btree
    AND index_bytes > 100000000
ORDER BY grp, index_bytes DESC )
SELECT reason, schemaname, tablename, indexname,
    index_scan_pct, scans_per_write, index_size, table_size
FROM index_groups;