Well most proxies you come across will just wrap it the way you are talking about. So yes, by looking at what is in the HTTP header can get you information on where a packet came from if behind a proxy. I think that this is how AntiOnline may show a Proxy Detected message under some user's posts.

There are also proxies called Anynomous Proxies, and I think that these take care to remove the original IP address. When you use these, it doesn't look like a proxy and AntiOnline will not be able to detect it.

There was a discussion a while back where someone asked why the proxy detected showed under their name... http://www.antionline.com/showthread...Proxy+Detected You may consider reading through the posts there since the information may be what you are looking for.

Also, Negative, a moderator here, put up a link to his proxy checker (on his server too)... It looks at the HTTP Headers and determines if it looks like you are behind a proxy or not. I think it will say "it doesn't look like you are using a proxy" if you use an anynomous proxy, since that information isn't passed on those... http://www.neg.be/php/checker.php