Spyjax: Using a:visited to test your history (2007) (ajaxian.com)

Spyjax doesn't work in modern browsers anymore after all the major vendors plugged the hole. That's why people are resorting to the timing method instead.

This trick still works:

https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Pri...

Although, the Google test on that particular page is currently not working.