There are aspects of a password you can check when someone is making or modifying one (length, types of characters, too close to the old one, etc.). Oracle even supplies a rudimentary password checking script in rdbms/admin. However, to do anything with password in dba_users, what is it you think you can validate? MIN1 and MIN2 both have aaaa as passwords, but you wouldn't know that from the encrypted value.

Code:
SQL> create user min1 identified by aaaa;

User created.

SQL> create user min2 identified by aaaa;

User created.

SQL> select username, password
  2  from dba_users
  3  where username like 'MIN%';

USERNAME  PASSWORD
--------- ------------------------------
MIN1      BD2419DC3285F69D
MIN2      7E255E3A0ED9B044