Determine Password Entered

There is no way to know which password was used within P5 plugin.
However, you may use this hook to retrieve password before P5 plugin has been triggered:

add_action( 'login_form_postpass', 'your_function', 9 ); // Priority = 9 to be called before P5
function your_function() {
    $password = $_POST['post_password'];
    [... do whatever you want...]
}