HttpWebResponse: How to determine success or failure

We have a web service that attempts to connect to Authorize.NET to process a payment, see below:

StringBuilder sb = new StringBuilder();
sb.Append("x_method=CC");
sb.Append("&x_type=AUTH_CAPTURE");
sb.Append("&x_delim_data=TRUE");
sb.Append("&x_relay_response=FALSE");
...
ASCIIEncoding Asc = new ASCIIEncoding();
byte[] postBytes = Asc.GetBytes(sb.ToString());

WebClient WC = new WebClient();
ServicePointManager.ServerCertificateValidationCallback = TrustAllCertificateCallback;
HttpWebRequest wrequest = (HttpWebRequest)WebRequest.Create("https://secure.authorize.net/gateway/transact.dll");
wrequest.Timeout = 300000;
wrequest.Method = "POST";
wrequest.ContentType = "application/x-www-form-urlencoded";
wrequest.ContentLength = sb.ToString().Length;

Stream lilriver = wrequest.GetRequestStream();
lilriver.Write(postBytes, 0, postBytes.Length);

HttpWebResponse wresponse = (HttpWebResponse)wrequest.GetResponse();
Stream rstream = wresponse.GetResponseStream();
Encoding enc = System.Text.Encoding.GetEncoding("utf-8");

StreamReader strRead = new StreamReader(rstream, enc);
string result = strRead.ReadToEnd();
wresponse.Close();
strRead.Close();
rstream.Close();

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How do I determine if the web service call was successful or not in terms of connectivity? I guess I could check the the contents of the result string in the code above to see if it's null or empty, but is there a better way?