Tips:

PM a Username if you want to check if a account is still on Yahoo!

A good way to test if account is still there pm the account name
yahoo will reply with message box like this

Yahoo! Messenger Error

The message for USERXXX was not
saved. Please verify the recipient's name
and try again. Thank you.

 

You will then know the account does on yahoo and you can use it

Pm yourself to check if your pm's are working
Sometimes you will get session expired then you know there not working.