Skype has done a good job of keeping its traffic from being identified or controlled by port. They use 443 and 80 for inbound , ports necessary for many other services and a random port per installed user for outbound. I don't think you can narrow down unless you can add those ports and a Skype server IP combination to identify the traffic.
One notable characteristic of Skype that I've seen is small packets on high UDP ports to a large number of unique Internet hosts. I haven't figured out a way to use NTA to quickly find hosts with that traffic profile--maybe someone here can help.
