Detecting IPv6 In the Webbrowser
From SixXS Wiki
Some Websites announce if you access them over IPv6 but not all of them. While most sites are still v4 only every once in a while you might come across a page which is already server over IPv6. Checking every page by hand is tedious and boring. The following plugin for Firefox shows the IP of the webserver and lights up in a nice green if it's an IPv6 address:
NOTE: The plugin only shows that the website has an IPv6 address, it doesn't show if the connection to the website actually was made using IPv6, thus making it a bit useless.