function spot_form(&$node) {

  $form['#theme'] = 'spot_form';

  ...

  return $form;

}

function theme_spot_form($form) {
echo 'hello world';
  $output .= '<div id="spot-form">' . "\n";
  $output .= form_render($form) . "\n";
  $output .= "</div>\n";
  return $output;
}