I am not very knowledgable on the technical aspects of IPs. I am a Detective in a small department that has to occassionally investigate computer crimes. If I receive an IP address,from a store in which a stolen credit card was used, of the computer that performed the transaction is there anyway to know if that computer was used as a relay without doing forensics on it.
Also, what is the best way for me to learn more without going back to college? Everything moves so quickly it's hard to keep up.
It may not take college, but it's going to take training. You're going to need to develop a toolkit, and that's going to take knowledge of what tools help you complete what.
You might want to get in touch with the US Department of Homeland Security (http://www.dhs.gov). I've heard that they might have resources (possibly including grants and other ways to assist you) to help get you up to speed.
As for your specific question, I think *some* measure of computer forensics is going to be necessary. And while such a PC *could* be a relay, it's far more easy to simply bring a fake card in and run it through a card reader. It depends on the exact purpose that computer plays within the retail establishment.