You can detect proxies via some clever PHP scripting, usually by examining the headers that proxies send out. Of course, not all proxies send this information for whatever reason, I'm behind a proxy at the moment but you won't see 'proxy detected' under my profile/username.

This script at Overdose explains a simple way to detect proxies. It's not foolproof by any means, but I think it's similiar to what JP uses.