theme.inc::template_preprocess_html checking visibilitiy of regions using has_render_content().

theme.inc

  // Add information about the number of sidebars.
  if (has_render_content($variables['page'], 'sidebar_first') && has_render_content($variables['page'], 'sidebar_second')) {
    $variables['attributes']['class'][] = 'two-sidebars';
  }
  elseif (has_render_content($variables['page'], 'sidebar_first')) {
    $variables['attributes']['class'][] = 'one-sidebar';
    $variables['attributes']['class'][] = 'sidebar-first';
  }
  elseif (has_render_content($variables['page'], 'sidebar_second')) {
    $variables['attributes']['class'][] = 'one-sidebar';
    $variables['attributes']['class'][] = 'sidebar-second';
  }
  else {
    $variables['attributes']['class'][] = 'no-sidebars';
  }