SQL> desc dba_users
 Name                                                  Null?    Type
 ----------------------------------------------------- -------- -------------------------------
 USERNAME                                              NOT NULL VARCHAR2(30)
 USER_ID                                               NOT NULL NUMBER
 [b]PASSWORD                                                       VARCHAR2(30)[/b]
 ACCOUNT_STATUS                                        NOT NULL VARCHAR2(32)
 LOCK_DATE                                                      DATE
 EXPIRY_DATE                                                    DATE
 DEFAULT_TABLESPACE                                    NOT NULL VARCHAR2(30)
 TEMPORARY_TABLESPACE                                  NOT NULL VARCHAR2(30)
 CREATED                                               NOT NULL DATE
 PROFILE                                               NOT NULL VARCHAR2(30)
 INITIAL_RSRC_CONSUMER_GROUP                                    VARCHAR2(30)
 EXTERNAL_NAME                                                  VARCHAR2(4000)

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as Todd has said,
passwrds are not just stored. it is hashed and
to access dba_users you need privs.
I dont get ur requirement.
if u want to check the existence of user, just check the username.!
u cannot compare passwords.
But if you are looking into
"Logging (connecting not querying from dba account) as 
another user USER_A and accessing his objects (from a 
dba account) without knowing password of USER_A" it is 
possible (something like "su" in unix).