How to check if an admin (sub)menu already exists?

Something like this should work. $handle should be the menu’s slug; set $sub to true to search submenus (defaults to top level menus):

function find_my_menu_item( $handle, $sub = false; ){
  if( !is_admin() || (defined('DOING_AJAX') && DOING_AJAX) )
    return false;
  global $menu, $submenu;
  $check_menu = $sub ? $submenu : $menu;
  if( empty( $check_menu ) )
    return false;
  foreach( $check_menu as $k => $item ){
    if( $sub ){
      foreach( $item as $sm ){
        if($handle == $sm[2])
          return true;
      }
    } else {
      if( $handle == $item[2] )
        return true;
    }
  }
  return false;
}

One last quick note: this should not be used before the menu is set. 'admin_init' is a safe bet for the earliest time to use this. I also added some code to leave gracefully if the menus aren’t set.