How to know if someone is Invisible/hidden or Offline on Gtalk.

Now if you want to fool the people and know about their online status even when they have logged in with invisible feature activated, then this is possible. There are two ways to do this.
You need to have a GPRS enabled mobile handset.
Download a free software called gotalkmobile on your handset and install it.
After the installation, log on to your GTalk account with your login details.
Now, this will show you the entire online friends list. It does not matter if they are online or invisible.
In this way you can catch your offline friends as well. The persons who are being shown as offline, are not present on GTalk.
And you are done above. You will notice those online (visible + invisible) as well as the offline friends.
Now u can even disturb that clever fools ;-)

So another way is that u just go Off the record with target user that is chats will now be not saved.
This can be only done when the target user is online.
Now when the target user is offline/hidden/invisible just try to send him/her the messages in chat. if the target is invisible but not offline then he/she can receive ur chats. but if the target user is offline then u'll receive an error did not receive your chat.

Source: Internet