I found this website it have a great software that allow you to detect invisible buddy. Below is the quote from that site.

"Are your Yahoo! Messenger friends hiding from you? Not any more! With BuddyCheck you can see if they are online even if they set their status to invisible.

Just add your friends to the contact list and their online status will be regularly checked. You will be notified when one of them really signs on of off."

It has free 10 days trials or you can purchase BuddyCheck now for only $19.95.