How do I get the values of checked checkboxes in a drupal? I'd like to display them in an alert everytime one of the checkboxes is checked. Below is what i am working with:

      $query = db_select('taxonomy_term_data', 't');
      $query
            ->fields('t', array('vid', 'name', 'tid'))
            ->condition('t.vid', 2, '=');
      $result = $query->execute();
      $options = array();
      foreach ($result as $row) {
            $options[$row->tid] = $row->name;
      };
      $form['mouth'] = array(
            '#type'     => 'checkboxes',
            '#weight'   => '11',
            '#options' => $options,
            '#prefix' => '<div style="background:#FF7F55;">',
            '#suffix' => '</div>',
            '#ajax' => array(
                'callback' => 'make_topic_chex_callback',
                'wrapper' => 'gunge',
                'method' => 'append',
                'progress' => array('type' => 'none'),
            ),
      );

function make_topic_chex_callback($form, $form_state) {
  $derps = $form_state['values']['mouth'];
  $commands = array();
  $commands[] = ajax_command_alert("Alert requested at " . $derps);
  return array('#type' => 'ajax', '#commands' => $commands);
}

In make_topic_chex_callback , I want to alert the values of the checkboxes in $form['mouth']