(Reliably) Calling A Function From A Dynamically Loaded Script

Is there a way to know when a function is available for me to call it
from a dynamically loaded a javascript?

I use this code to load the include.js file and then I call
testIncludeFn() from it:

<code>

var html_doc = document.getElementsByTagName('head').item(0);

var file = document.createElement('script');
file.setAttribute('language','javascript');
file.setAttribute('type','text/javascript');
file.setAttribute('src','/include.js');
html_doc.appendChild(file);

// here I would like to be able to wait for the script to become
available

testIncludeFn();

</code>

It works in IE and sometimes I get it to work in FF 1.5, but I would
like to have a way of reliably calling testIncludeFn().
Can I pool somehow the DOM to see if the load completed. How?