Here are few links where we can check our friends status like invisible, offline or online. Well, not only friends but we can check with anyone's yahoo ID if we do know the email Id.
1. ydectector.com
2. detectinvisible.com
3. imvisible.com
4. invisible-scanner.com
5. statusdetect.com
There are other lots of sites and blog you can google. But I think these few links would enough.