If you access to the proxy server, set X-fowarded-to to delete
http://www.squid-cache.org/Doc/config/forwarded_for/. If not, go to any proxy detecting website (through google) and see if it can detect that you are behind a proxy. If X-forwarded-to is deleted, the other side will only see the IP of the proxy and not your IP. If you are still not sure, use something like the TOR network.