[How To]Check Invisible really

I see some websites can check invisible status yahoo.
Sometimes i make some code to filter content on those sites to find yahoo status
But i want to know how those sites can do that
Ex : website http://www.vngrabber.com/

Can we use socket, or secret link to yahoo like "http://opi.yahoo.com/online?u=[username]&m=g&t=2 or … to do that