This is very, very simple. First, you'll need to download CommView.
http://www.tamos.com/download/main/index.php
It's up to you if you want to use a cracked version.
Step 1: Call the person on Skype. Have CommView running.
Step 2: While you're on call with the victim, go into CommView and see which task/process/connection is using the most data. The IP of the victim is the one that is doing the above. ^^^
This is true so long as Skype is the only thing running that uses a lot of bandwidth.