ASP.NET MVC 4 - Proper way to respond in HTML, JSON, or XML

You have several options here.

First - differentiate your requests by their verbs, and have a convention, such as all GETs receive html, while all POSTs receive json. Controller will look like this:

[HttpGet]
public ActionResult Index()
{
    return View();
}


[HttpPost]
public ActionResult Index()
{
    return Json();
}

And of course its a client-side matter to make a request with correct method.

Second - introduce a parameter. Say by default you are sending html, but if ajax call appends a parameter isJson - give it json response:

public ActionResult Index(bool? isJson)
{
    if (isJson.HasValue && isJson.Value)
    {
        return Json();
    }

    return View();
}

Third - differentiate request by the mechanism behind them. In your case it seems that all ajax calls, and only them, should be served by json. Then you can use Request.IsAjaxRequest() method:

public ActionResult Index()
{
    if (Request.IsAjaxRequest())
    {
        return Json();
    }

    return View();
}

And of course it is possible to combine these methods - say send json response only to POST requests by ajax.

May try to resolve this with request AcceptTypes?

 public ActionResult Index()
        {
            if (Request.AcceptTypes.Contains("application/json"))
            {
                //
            }
            else 
            {

            }
            return View();
        }