Yes,Buddy Spy scan is correct.
It uses a system that sends carefully crafted packets, and listening to their responses.
Doing so you can see if your friends are using the stealth or invisible mode of Yahoo Messenger.
I recommand using this settings to ensure complete succsess of your scan:
Connection Settings
YMSG server: scs.msg.yahoo.com
YMSG Port: 5050
Cam Server: wc1.msg.vip.re2.yahoo.com
Scan Type
Avatar Packet