Many people now days like remain on an invisible mode on GTalk.Even today when I was online this morning my friend was invisible on Gtalk. I wanted to chat with him but I didn't know he was online to.It was later in the dy that I came to know that he was online.
One of my friends once told me that there is a way to detect who is invisible on GTalk so today I searched For the trick and found it on Vishal's blog.

The trick goes like this:

1. Set the chat as off the record with target user.

2. Open the chat window of person, whom you want to check if Invisible or Offline. Go to Options and click "Go Off Record".

3. Now send message to that user

4. If you get the message that "The user is offline and can’t receive messages right now " that means the user is offline.

5. But if you didn’t get any message that means User is online and is in invisible mode..:)