show all ROLES granted to a USER:

postgres-# select roleid, rolname, usename, member  
postgres-# from pg_auth_members 
postgres-# join pg_roles on pg_roles.oid = pg_auth_members.roleid 
postgres-# join pg_user on pg_user.usesysid = pg_auth_members.member  
postgres-# where usename = '...' ;