Yahoo messenger allows stealth mode using which you can set your online status as “offline” even when you are logged in to yahoo messenger. People use it to avoid chat when they are busy or to block out spam messages.
1. But there is a hack which allows you to see whether someone is really offline or not. The easiest way is to directly acess the online status image using the URL - http://opi.yahoo.com/online?u=userid&m=g&t=2 (replace userid with the actual user id).
2. The second way is to start a voice chat with the userid you want to check. If you don’t get an error it indicates that the user is indeed online, but in invisible mode. This works only in previous versions of Yahoo messenger.
3. There is another software which checks if some friend is invisible