EXPLAIN 
SELECT * FROM TABLE
WHERE fk_id=x;

"Seq Scan on table  (cost=0.00..422.09 rows=14791 width=66)"
"  Filter: (fk_id = 33)"

"Index Scan using table_fk_id_idx on table  (cost=0.00..48.82 rows=108 width=66)"
"  Index Cond: (fk_id = 32)"