You would have to check the HTTP response headers to see if such a header is send by the server. *https://developer.mozilla.org/Tools/Web_Console

Hmm, Is there any add-on that will make it simple to show :) ?

There is one to prevent click jacking, however if you are interested in makeing one: *[https://developer.mozilla.org/en-US/Add-ons MDN Add ons ] *[https://developer.mozilla.org/en-US/docs/HTTP/X-Frame-Options] *[http://stackoverflow.com/questions/7950184/is-there-a-client-side-way-to-detect-x-frame-options] Clickjacking add ons *[https://addons.mozilla.org/en-us/firefox/addon/no-clickjacking/] *[https://addons.mozilla.org/en-US/firefox/addon/clickjacking-defense-declar/]