How can I check to make sure a window is being actively used, and if not alert the end user that they are about to be logged out?

<meta http-equiv="refresh" content="900;url=http://example.com/login"/>

<input type="text" onClick="doSomething;">

Mozilla way:   element.addEventListener('click' ...)
Microsoft way: element.attachEvent('onclick' ...)

setTimeout(timeoutWarning, 240000);

function timeoutWarning() {
  if(confirm('You have been idle for a while.  Would you like to remain logged in?'))
    window.location.refresh();
}