This is old method how you can find Main ID through alias ID of Yahoo Messenger. This meant if someone online and they only use Alias Id to chat with you, you can find the real ID aka Main ID of them just using of Buddy Spy Invisible Checker.
First just check if the ID online or invisible or offline using Buddy Spy. If they are online or invisible, you can see the main id from alias ID on the footer of buddy spy software. It should be like this: if alias id has a name johnbiden and the main id is jhonchow, the status in the footer of buddy spy would be like this : johnbiden is alias of jhonchow.