hello
i have an iframe in my homepage.

if there any function in javascript to findout if the iframe has not loaded and if not loaded load another src.

This can only be achieved if the iframe have it's url on the same domain that your main page.
IE: the main page url is http://www.whatever.com/index.html
If the iframe points to something else than http://www.whatever.com/..., you will not be able to detect the loading.

Then, for the fun part, in your main page:

<script type="text/javascript">
function iframeLoaded(){
  alert('the iframe is loaded');
}
</script>
<iframe id="ifSub" name="ifSub" src="http://www.whatever/iframe.html" onload="javascript:iframeLoaded();"></iframe>

Just 1 note: every time the iframe will load a page, the event onload will trigger, meaning that it won't be only on the initial load.
If you want to avoid that, use a counter value to restrict the effect to the first call.