title: How to detect the visibility of a cross-domain Iframe?
body: <p>There are companies that claim to be able to detect the visibility of an iframe without access to the embedding page. Two I found are <a href="http://spider.io/blog/2011/10/demonstration-screencast-verifying-that-display-ads-are-visible-from-within-iframes/" rel="nofollow">spider.io</a> and <a href="http://blog.comscore.com/2012/08/accreditation_big_step_on_multi-platform_journey.html" rel="nofollow">comscore</a>, both of which say their technology is patent pending. Any ideas how they might be doing this? </p>