Fire an event when an window property is ready to be used. This assumes that an external script will add the new property on the window object.

is-it-loaded-yet.js

// This uses Google tracking as an example
var interval = setInterval(function () {
  console.log('searching..')
  if (window._gaq) {
    _gaq.push(['_trackPageview', 'Something']);
    clearInterval(interval);
    console.log('stop');
  }
}, 10);