This is a small trick to view the status of any Yahoo! Instant Messenger user to check if he or she is online or offline even if they are not on your contact list or have blocked you. You can do this without signing in or logging into Yahoo! Messenger on your computer.

Type the following URL in your web browser replacing yahooid with the user’s actual Yahoo! Email address/IM id:

http://opi.yahoo.com/online?u=yahooid&m=g&t=2

If the contact is online, the following logo will be displayed

I have tested this and it works like a charm.