Awesome article on understanding postgres performance had this little nugget, which gives you the tables in your database with the percentage of time they use an index:

SELECT 
  relname, 
  100 * idx_scan / (seq_scan + idx_scan) percent_of_times_index_used, 
  n_live_tup rows_in_table
FROM 
  pg_stat_user_tables
WHERE 
  seq_scan + idx_scan > 0 
ORDER BY 
  n_live_tup DESC;