Recently I observed a small thing in GTalk that can easily tell you whether he is actually invisible or offline. Just ping the user that you want to know about his status, if the response doesn't contain anything in red and the response is something similar to the image below then the user is in invisible mode.
If the response is something similar to that of the image shown below(if you get the response in red color) then he is actually Offline.
The messages that you get in turn as response in the above two cases are different, which helps us to know the status of the user. This seems to be a very simple trick to check ... :) I have been using this successfully for a while to check my friends status.
