Various online resources offer proxy lists claiming that they offer various levels of anonymity. I'm trying to find out how this can be programmatically tested. I suspect the answer lies in headers. Would testing a proxy's security by running a sender/receiver and seeing what headers come through the proxy on the receiving end be a good approach?
EDIT: One good resource I found so far mentioning proxy headers: http://meatballwiki.org/wiki/AnonymousProxy