I recently wrote about how you can be invisible on Gtalk messenger. Now imagine if any of your friend has the Labs version of GTalk installed, then their is a possibility they can be invisible, and it is difficult to find if they actually are online and invisible, or offline.
But there is an easy way to find out if your friends are online or invisible on GTalk:
1> You need to be logged in to your account,
2> Then send a message to any of your friend who you doubt is invisible. If they are actually offline then you get a message like this :
If the person is invisible then no such message will be seen.