How to determine whether window.onload is used multiple times

I believe everyone has used window.onload. It is executed when the window is loaded. How to judge if window.onload is used multiple times? Check out the code below

 <script>function onloadCallback(){//Todo}if (window.onload) {var onload_random = 'onload'+Math.random();window[onload_random] = window.onload;window.onload = function (){window[onload_random]();onloadCallback();};}else {window.onload = function (){onloadCallback();};}</script>