You can use the code in this tip to make web requests to a remote HTTP server and read the response:

try
{
   // The URL http://www.microsoft.com/en/us/default.aspx 
   // is used here purely as an example.
   // Substitute your own URL as needed.
   Uri uri = new Uri("http://www.microsoft.com/en/us/default.aspx");
   System.Net.HttpWebRequest httpWebRequest = 
      (System.Net.HttpWebRequest)   
      System.Net.HttpWebRequest.Create(targetUri);

   if ((httpWebRequest.GetResponse().ContentLength > 0))
   {
      System.IO.StreamReader str = new System.IO.StreamReader(
         httpWebRequest.GetResponse().GetResponseStream());

      Response.Write(str.ReadToEnd());
      if (str != null) str.Close(); 
   }
}
catch (System.Net.WebException ex)
{
   Response.Write("File does not exist.");
}