public static bool PortInUse(int port)
{
    bool inUse = false;

    IPGlobalProperties ipProperties = IPGlobalProperties.GetIPGlobalProperties();
    IPEndPoint[] ipEndPoints = ipProperties.GetActiveTcpListeners();

    foreach (IPEndPoint endPoint in ipEndPoints)
    {
        if (endPoint.Port == port)
        {
            inUse = true;
            break;
        }
    }

    return inUse;
}

static void Main(string[] args)
{
    HttpListener httpListner = new HttpListener();
    httpListner.Prefixes.Add("http://*:8080/");
    httpListner.Start();

    Console.WriteLine("Port: 8080 status: " + (PortInUse(8080) ? "in use" : "not in use"));

    Console.ReadKey();

    httpListner.Close();
}