A friend of mine was doing a certain type of marketing on a certain site using public proxy lists he found online. Recently his ISP got some letters from the certain site about his activities. He just didn't realize how many public proxies leak your IP in the request headers. If the proxy isn't completely stealthed, there's not much point using it with many larger sites. I showed him a few different ways to check but he asked me to put something together that was as simple as possible, just go to the URL and get an answer. I totally spaced on doing that till last night. Just go to http://orlyproxy.com/ when you have a proxy active and it'll tell you if your proxy is being stealthy or obvious with headers you might need to worry about (if any).