Getting list of blocked websites in proxy server

I want to have a diagnostic to ensure that my application will not be blocked to access the remote server because of the proxy setting. Is there a way to get the list of a blocked websites in proxy server from the client (.NET)? I suppose I could use WebRequest and try to call simple echo method in the server and whether I get a 403 or not but wondering if there is other way to do that.

Thanks.

This fully depends on your proxy. There is no standard way to achieve this, but your proxy may implement an interface that allows you to retrieve such information through a management API.