I have created a PoC to try to get full anonymous proxies. This script discovers if your IP is full changed and anonymous using a proxy. It works via NSE by NMAP.
I wrote one post about it in Spanish here: http://www.securityartwork.es/2013/05/14/a-vueltas-con-la-deteccion-de-proxys/
The PoC source code here: https://github.com/vicendominguez/http-open-proxy-anon