Question

The web app I need to test is made up of frames and has an HTML comment (<!-- some text -->) at the end which will be present when the page finishes loading.

Answer 1

You could get the pagesource and check if the comment is there

 _driver.Navigate().GoToUrl("http://mysite.com");
  var source = _driver.PageSource;
  //check the source of the page to see if the commet is there.