If we need to validate the front end user password is working fine or not then we can fire the below query to check the authentication of password.

Login to Database as Apps user.

SQL> select fnd_web_sec.validate_login('GUEST','welcome') from dual;

FND_WEB_SEC.VALIDATE_LOGIN('GUEST','WELCOME')
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Y

If the output is Y  then password is correct and if it is N then it invalid.