// create the request
HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;

// instruct the server to return headers only
request.Method = "HEAD";

// make the connection
HttpWebResponse response = request.GetResponse() as HttpWebResponse;

// get the status code
HttpStatusCode status = response.StatusCode;

if this gives you The remote server returned an error: (401) Unauthorized because of some servers configuration then drop the request.Method = "HEAD"; line

source:  stackoverflow - Miller answer + my comment on it