How to test Headers in a ReverseProxy?

I am trying to unit test the following code:

func(h *Handler)Forward(w http.ResponseWriter, r *http.Request) {

    url, err := url.Parse("http://test.com")
    if err != nil {
       return
    }

    reverseProxy := &httputil.ReverseProxy{
        Director: func(r *http.Request) {
            r.URL.Host = url.Host
            r.URL.Path = "/"
            r.URL.Scheme = url.Scheme
            r.Host = url.Host
            r.Header.Set("X-Forwarded-Host", r.Header.Get("Host"))
        },
    }


    reverseProxy.ServeHTTP(w, r)
}

I am not able to figure out how to test whether headers are being modified by the Director function. How do we test headers in a reverseproxy in Go?