Determining whether the window has loaded without using any global variables

I need to find out if the window has loaded or not.

I want to create a checkLoaded function that will return true or false to indicate this, based on when I call it.

<html>
<head>
  function checkLoaded(){
    //alert true if window is loaded or alert false
  }
</head>
<body onload="checkLoaded()"> <!-- This should alert true -->
   Loding window.
   <script>
       checkLoaded();// this should alert false;
   </script>
</body>
</html>

I don't want to use a global variable that I set when the window loads.

Is there any way that I can check the window object's status, perhaps a property?

I don't want to use jQuery or any other external libraries.