How do you know if user_login_submit was successful in Drupal 7? Based on the answer from this question How do I programmatically log in a user?

I know how to login in a user, but is there a way to test if user_login_submit was successful or not? It does not have a return value.
You can check if user is logged in or not by using global $user variable, if user is not logged in then $user->uid will be 0 for anonymous users.