HttpWebRequest req = (HttpWebRequest)WebRequest.Create(@"http://sourceforge.jp/frs/redir.php?m=iij&f=%2Fnkf%2F48945%2Fnkf-2.1.1.tar.gz");
req.Method = "HEAD";

HttpWebResponse res = (HttpWebResponse)req.GetResponse();
Console.WriteLine(res.ContentType);

＞application/x-gzip

そんな難しいもんでもあるめえ