Proxy Scanner

I''m interested in making a proxy scanner in C sometime soon, and I was wondering if anyone has been able to make one that works well at detecting various types of proxies e.g(anon or non-anon). I figure for instance I can attempt open a streaming socket with an arbitrary ip on port 80 or 8080 and send the http get request to retrieve a certain resource on my server and if a successful http response is received from the remote location then it passes. However, I wonder about how long is a reasonable time to wait for a response from the remote location before deciding whether it is or not a proxy? Also what methods are there to determine whether the proxy is or is not an anon? Again, I think that info can be retrieved from an http request header that is sent from the proxy to the remote resource which if the remote resource is my own server it shouldn''t be hard to get, but what if the remote resource is say yahoo.com? Is there another method to determine the type of proxy? I imagine the coding will be trivial once I can figure out the most efficient methods.