Many people already blogged about this article. I made the same experiment. It works.
If you want to find say firstname.lastname@example.org is invisible or not.
Open your gtalk app and type this email address in top search textbox
Click the profile, it opens in new window as usual
Click on the down arrow button in the top right corner of the window, and you can see Go off the record link there.
And type any chat, say “hi”
You will get a red text showing the “person is offline , can’t receive the message right now ” implies the person is offline
If this message does not appear, it means the user is invisible
To know more about gmail and gmail chat visit my post series