Use the following query to show all tables without primary keys:

SELECT
  nspname AS schema,
  relname AS table
FROM pg_class
  LEFT JOIN pg_constraint ON
    pg_constraint.contype = 'p' AND
    conrelid = pg_class.oid
  JOIN pg_namespace ON
    pg_namespace.oid = pg_class.relnamespace
WHERE
  relkind = 'r' AND contype IS NULL;