我如何找到window.onbeforeunload上正在运行的事件？

Question

我如何找到window.onbeforeunload上正在运行的事件？例如刷新，返回，浏览器关闭等

<script language="JavaScript" type="text/javascript">
  window.onbeforeunload = confirmExit;
  function confirmExit()
  {
    alert('here i need to show which event is working')
    return "";
  }
</script>