System.Web.Mvc.ReflectedActionDescriptor descriptor = filterContext.ActionDescriptor as 

System.Web.Mvc.ReflectedActionDescriptor;

if (descriptor != null)

{

    Type actionType =descriptor.MethodInfo.ReturnType;

    if (actionType.Equals(typeof(ActionResult)))

    {

        if (actionType.IsSubclassOf(typeof(JsonResult)))

        {

            Response.Write("JsonResult");

        }

        else if (actionType.IsSubclassOf(typeof(ContentResult)))

        {

            Response.Write("ContentResult");

        }

        else

        {

            Response.Write("ViewResult");

        }

    }

    else

    {

        if (actionType.Equals(typeof(JsonResult)))

        {

            Response.Write("JsonResult");

        }

        else if (actionType.Equals(typeof(ContentResult)))

        {

            Response.Write("ContentResult");

        }

        else

        {

            Response.Write("ViewResult");

        }

    }

}