Through as in get through someone elses proxy and get access to there internal network
hmm word?

Ok first you need to know if the proxy is a local one or one dat is being accessed via the internet. Getting through one that is being accessed via the internet is useless and irrelivant. but the local one yeah.. that can be aranged.

Two words: log files.

IF you can get hold of the log files of the proxy server, you can get the IP Addresses of the machines logged on to that proxy server.

IF you want to know how to get the login files.. THAT is a LOOONNGG story coz you are back to square one.. if you know what i mean, IF.. you know what the next step is, you would know what i mean..