I wrote quite a popular blog post on how to check if a user to your site is logged in to GMail, Facebook, Twitter and Digg: https://grepular.com/Abusing_HTTP_S

I wrote quite a popular blog post on how to check if a user to your site is logged in to GMail, Facebook, Twitter and Digg: https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Private_Information https://grepular.com/Abusing_HTTP_Status_Codes_to_Expose_Pri... GMail doesn't seem to be working any more though. But the concept is still useful to learn.