static void Main(string[] args)
    {
        String url = "https://www.example.com";
        HttpWebRequest request = WebRequest.CreateHttp( url );
        request.ServerCertificateValidationCallback +=  ServerCertificateValidationCallback;
        using( HttpWebResponse response = (HttpWebResponse)request.GetResponse() ) { }
        Console.WriteLine("End");
        Console.ReadKey();
    }

    private static bool ServerCertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
    {
        if (sslPolicyErrors == SslPolicyErrors.None)
        {
            Console.WriteLine("Certificate OK");
            return true;
        }
        else
        {
            Console.WriteLine("Certificate ERROR");
            return false;
        }
    }