Proxy IPs would be a good bet if the servers' users were that way inclined. If you have specific IPs then run a port check of them (I use linux umit) to see if there are any open ports that could be used for any purpose such as http, ftp. If so, it's a fair bet they could also be used as proxies.
I monitor all proxies that hit my servers (check the headers). Many are harmless and are let through, others are blocked.