Yahoo Messenger is the most common and liked messenger by us and we can easily choose our status to either Online or Invisible. It gives us the option of hiding from our buddies and tracking them was very difficult. But now we can easily check whether the person is really offline or just hiding his ass from us.
There may be many ways but what I found the best is to use some sites that offer this service of tracking our Yahoo buddies. Below is the list of sites that I found of great help (though you should not completely rely):
Just give a shot to these sites and surprise your friends who pretend to be offline. Let me know if these worked for you and if you know some other technique as well then feel free to share with me.