If you want to be in invisible state without can be noticed by others, you can set at the Preferences / Privacy / Login as invisible. Using that option, you will be invisible all time. Even at the first time you logged in you will be invisible.
Use the Yahoo Message Archive Decoder (look at http://www.ikitek.com/products/Yahoo-Message-Archive-Decoder.html)
now also with decode of message archive in their website i tried it and it can view other user messages
but i found a site which will tell you how to tell if someone is really offline or invisible.. it’s http://yahoo-invisible.ikitek.com
it works for me!