If you doubt whether your buddy is online or not, hiding or not, you can use BuddySpy. You can try (download) here.

I often used xeeber.com to detect my buddies, but BuddySpy is the desktop software. You can use it if xeeber.com is down.

Buddy Spy allows you to monitor and keep track of what other Yahoo Messenger! Users are doing, even if they are in Invisible or Stealth Mode. The program shows if the user is online, what chat room they are in (if any) and if their web cam is online. It is able to do this by connecting to Yahoo! Messenger’s servers and using its YMSG protocol. Sending carefully crafted packets, and listening to their responses.

[Read the rest on (it)gossips network: buchin]