Yahoo messenger had a cool feature called invisible mode. Sometime people misuse this functionality which so annoying. There are couple of solution to hack whether the invisible one are actually online.
Let me explain few methods to check whether invisible one is actually online or not? One which is very official way that provided by Yahoo itself by checking the profile directory. Suppose your target profile be `xyz`. Open the URL http://profiles.yahoo.com/xyz in your browser. The page entitled with Yahoo status. The page will display the information about the account like name, sex, age, occupation etc.
Another way to check the user is invisible is http://mail.opi.yahoo.com/online?u=username&m=a&t=1
This will show the status of the user, If the status is 00 then user is invisible and 01 if the user is online.
Another way which is not generic one. Lot of providers are giving status message from yahoo. One such provider is Yahoo status checker. You can provide the username on the textfield provided and click on submit button. The application will provide you the status of yahoo messenger.