登録ユーザの情報を確認

select * from pg_user

dbname=# select * from pg_user;
 usename  | usesysid | usecreatedb | usesuper | usecatupd |  passwd  | valuntil | useconfig
----------+----------+-------------+----------+-----------+----------+----------+-----------
 postgres |       10 | t           | t        | t         | ******** |          |
 admin    |    16407 | t           | t        | t         | ******** |          |
 support  |   152359 | f           | f        | f         | ******** |          |
(3 rows)

あるいは

\du

dbname=# \du
                       List of roles
 Role name |            Attributes             | Member of
-----------+-----------------------------------+-----------
 admin     | Superuser, Create role, Create DB | {}
 postgres  | Superuser, Create role, Create DB | {}
 root      | Superuser, Create role, Create DB | {}
 support   |                                   | {}