Identify Proxy Headers like HTTP_X_FORWARDED_FOR, HTTP_VIA, etc...
Known proxy host names like cache, server, proxy etc...
Match ip with known proxies (companies can easily code a spider to grab all public proxies) or running them against a blacklist like SORBS
Port scanning known proxy ports, 3128, 80, 8080, 1080 for a open connection.
GeoIP - Geolocation (for commerce)
More advanced detection uses
Softblocking known ip address classes (block ips from known providers or webhosts like theplanet, etc... while semi-whitelisting only ISP IP address blocks like comcast/time warner ip range)
Java or ActiveX / user side scripts to grab your true IP.
Cookies - depending on if the proxy relays information or keeps it
Bottom line is that you can pretty much tell whos using a proxy or not, Elite or not. Elite is just a overused hyped up phrase that providers are using now in days. Back in my days, elite proxies referred to a class of proxies that did not spill your true ip nor give any proxy headers as well as not being able to DNS them, so you could not resolve them to a hostname, because hostnames give off alot about if youre using a proxy or not.
In reality, almost all their proxies are going to be abused. Thats the reason why people get them. There aren't enough ip4 addresses to go around to keep all those IPs off the public or private proxy lists, so eventually ALL proxy ips are going to be burnt out, while the blacklists grow. The only true way to have 'elite' proxies is basically do it illegally (through viruses/rat) PERSONALLY, because if you don't there is no way of knowing who your provider handed them out to before.
