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.





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This one is simpler
http://invisible-scanner.com/
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This website works all the time: DetectInvisible.com and you can download the avatar too.
how can i see who’s apying me with buddy spy?
Yahoo invisible detect and scan is what we do. We are the best yahoo invisible checkers. http://www.statusdetect.com . We were born to do this. Auuuuuggghhhh Auuuuuggghhhh Auuuuuggghhhh
http://www.ydetector.com is realy good