web problem...check proxy validation

Question

Hi all,
I have a problem , how can I know if the proxy is working or not from c# app?
there is a lot of difficults met me
if you want to see if there a response from the requested site..this will be failed
because the ISP's can make response to the request (alternative page contain block reasons)  so c# will give you that response state is ok. and if you will compare the request site with the urlresponse it will be fail because when you request http://www.google.com it will be using proxy be http://www.google.co.uk if it is UK's proxy and so on
so how can i derermin if the proxy works or not????
please help me and the answer must be studied carfully before posted here....

thanx alot..