This is a diff method and I found it quite accidentally..
While running an nmap scan over the network, came to notice admdog on port 5101.
Did a quick Google on it..
and came to know that it was the yahoo messenger port.
So even if your friend is invisible running an nmap (provided you know the ip 🙂 ) you can get to know whether he is online.
But I found this hack to be pretty useful though even better methods are there to check on the Messenger Status.