how to inform client that data get from server comes from Web API cache

Hello

I have created a web API A that get some JSon data from another Web API B

I want to cache this Json data on my web API A as the data from B will not change frequently and that lot's of call will be done on the WEB API A to get this Json data.

So I have implemented Cache on the server like below:

	public async Task<HttpResponseMessage> getFedCustomers()
        {
            try
            {
                using (var client = new HttpClient())
                {
                    HttpResponseMessage response = null;
                    var cacheKey = "fedcache";
                    var cTime = DateTime.Now.AddMinutes(double.Parse(ConfigurationManager.AppSettings["CacheDelayInMinute"]));
                    var cExp = System.Web.Caching.Cache.NoSlidingExpiration;
                    var cPri = System.Web.Caching.CacheItemPriority.Normal;
                    string responseBody = "";
                    JavaScriptSerializer Jser = new JavaScriptSerializer();

                    if (HttpContext.Current.Cache[cacheKey] == null)
                    {
                        client.BaseAddress = new Uri(ConfigurationManager.AppSettings["IntegrationServiceBaseAddressURI"]);
                        client.DefaultRequestHeaders.Accept.Clear();
                        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

                        response = await client.GetAsync(ConfigurationManager.AppSettings["FedListURI"]);
                        response.EnsureSuccessStatusCode();    // Throw if not a success code.
                        responseBody = await response.Content.ReadAsStringAsync();
                        var test = Jser.DeserializeObject(responseBody);
                        HttpContext.Current.Cache.Insert(cacheKey, test, null, cTime, cExp, cPri, null);
                    }
                    else
                    {
                        var CacheBody = HttpContext.Current.Cache[cacheKey];
                        response = this.Request.CreateResponse(HttpStatusCode.OK, CacheBody, "application/json");
                    }

                    return response;
                }
            }
            catch (Exception ex)
            {
                string errorMsg = "";

                if (ex.InnerException != null)
                {
                    if (!string.IsNullOrWhiteSpace(ex.InnerException.Message))
                    {
                        errorMsg = string.Format(ConfigurationManager.AppSettings["ProcessFailed"], "getFedCustomers", ex.InnerException.Message);
                    }
                    else
                    {
                        errorMsg = string.Format(ConfigurationManager.AppSettings["ProcessFailed"], "getFedCustomers", ex.Message);
                    }
                }
                else
                {
                    errorMsg = string.Format(ConfigurationManager.AppSettings["ProcessFailed"], "getFedCustomers", ex.Message);
                }

                return Request.CreateResponse(HttpStatusCode.BadRequest, errorMsg, "application/json");
            }
        }

Cache seems to work properly but on client side I never know if data comes from cache or the call to the Web API B.

I have tried to set the HttpStatusCode.NotModified in the else when we return cache data but no data are returned in that case.

I want that all clients get the same data from the server, so if data is in cache it will be cache data otherwise a call will be done to the web api B. 

I have also tried to implement [CacheOutput( ServerTimeSpan = 60)] Strathweb.CacheOutput.WebAPi2 but looking at fiddler each user start by a requestStatus 200 then 304. I should expect the first one at 200 and the other at 304 due to the delay that I set to some minutes.