Here's how you tell. First, you need to download a free program call Yahoo Messenger here. If you already have a My.Yahoo account you are set, just download this program, put in your id and then add me as friend.
My ID is es7383. That is it.
Simple wasn't it? Now all you have to is open the program and if
I am one next to my name will be a Yellow Smiley face, like this 
. Now you can
chat (conference) with me, send offline messages if you just
missed me or even do a voice chat.
It integrates very well with My.Yahoo.com so you can custom configure it to tell you when you have new email (on you yahoo account), get news that you care about, stocks if you are interested, sports scores on only the teams you are interested in and even tell when your friends are online, like ME. It is great.