How to determine a 404 response status when using the HttpClient.GetAsync()

You could simply check the StatusCode property of the response:

static async void dotest(string url)
{
    using (HttpClient client = new HttpClient())
    {
        HttpResponseMessage response = await client.GetAsync(url);

        if (response.IsSuccessStatusCode)
        {
            Console.WriteLine(response.StatusCode.ToString());
        }
        else
        {
            // problems handling here
            Console.WriteLine(
                "Error occurred, the status code is: {0}", 
                response.StatusCode
            );
        }
    }
}