Gettting uid from inside hook_form_alter()

I'm inside module_form_alter(), switch ($form_id), case 'user_profile_form': and trying to get the uid of the user being edited. I can see several places within $form that it is available, but I'm unsure of whether they will always be there or what the best practice is for finding the uid.

Just as an example (and this isn't at all what I'm doing but it gives the scenario) here is some code that I'd like to have filled in. What should $form['#parameters'][2]->uid be?

module_form_alter(&$form, $form_state, $form_id) {
  switch ($form_id) {
    case 'user_profile_form':
      // Set $user_id equal to the uid of the user who's profile form is being displayed.
      $user_id = $form['#parameters'][2]->uid; // Replace this with ???
      break;
  }
} // function module_form_alter

This is what I'm using now, but I'm not certain that it is best practice and want to consult people who aren't doing it for their first time.

I greatly appreciate your help. :)