learn about different protocols and the data that their packets contain. you can find out about the protocols from here : http://www.faqs.org/rfcs/
that way you know what you are looking for when you look at the packets...
"Mrs. Jones, I'm sorry to inform you, but we've run the tests, and it appears that you have XP. Now don't cry - it's bad, but it's not a death sentence. Modern science has advanced in recent years, and it's now possible to live a reasonably happy life with XP. And there's a survivor's group that you'll want to meet as well."