Method to determine whether window.onload is used multiple times

Window.onload, which you've probably used before, is executed when the window is loaded. How do you determine if you use window.onload multiple times? Take a look at 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>