This really got my attention. I was questioning the same a while back so i started googling again and i ended up with this.
It might not be what you're looking for, but you'll get an idea on how the anonimity sort of works
Code
-Transparent Proxy is a normal proxy server which actually does not improve your online privacy.
Transparent Proxy can provide one benefit by caching frequently accessed pages and this process reduces download time and speed up access for the client.many companies also use this system for filtering purposes.
A transparent proxy will display the following information:
REMOTE_ADDR = Proxy's IP address
HTTP_VIA = Proxy's IP address
HTTP_X_FORWARDED_FOR = Your real IP address
-An anonymous proxy offers the caching benefits of a transparent proxy, while hiding your IP address by replacing with either the proxies IP, or a random address.
Anonymous Proxy server does not send HTTP_X_FORWARDED_FOR variable to host, this improves privacy since your IP address cannot be logged.
An anonymous proxy will display the following information:
REMOTE_ADDR = Proxy's IP address
HTTP_VIA = Proxy's IP address
HTTP_X_FORWARDED_FOR = Proxy's IP address or a random ip
-An elite or high anonymous proxy makes your system look the same as a non-proxied system. This provides the best security and there's no way to detect an elite proxy.
High anonymity Servers don't send HTTP_X_FORWARDED_FOR, HTTP_VIA and HTTP_PROXY_CONNECTION variables. Host doesn't even know you are using proxy server and of course it doesn't know your IP address.
A highly anonymous proxy will display the following information:
REMOTE_ADDR = Proxy's IP address
HTTP_VIA = blank
HTTP_X_FORWARDED_FOR = blank
Again, its not an answer to your question, but it can help you pointing in the right direction of the answer
EDIT: I THINK that a website that checks the proxy just tries to resolve:
REMOTE_ADDR
HTTP_VIA
HTTP_X_FORWARDED_FOR
But I'm a total noob if it comes to this so I dont know
