Find Who is invisible in Gmail chat

So it has been always a intresting query to me that how to find out a person invisible in gmail chat. Tough GTalk doesnot provide the facility of being invisible but GMail do. Now What you need to do:
Step 1.
Sign in with your GTalk.
Step 2.
Seach the contact by typing his/her name in the search box.
Step 3.
Once you got the contact click the entry to open chat window.
Step 4.
Click the drop down arrow under close button and "Go off the record".
Step 5.
Now send a message to him/her.
Step 6.
if the message is saying .@gmail.com is offline and can't recieve messages right now.' then the user is logged off. If the message doesnot appear he is invisible.
Note: This trick works most of the time but may fail to work if he/she is using Google talk with third party installation tools like mobile chat applications and other ones as Meebo or Digsby.