JSF can answer this question by inspecting request headers or checking the PartialViewContext.isAjaxRequest method. The request headers that provide information about the request type are Faces-Request and X-Requested-With. For an AJAX request, the Faces-Request header will have the value partial/ajax, while the X-Requested-With request type will have the value XMLHttpRequest (in JSF 2.2, X-Requested-With doesn't seem to work; however, for the sake of completeness, you can test them again). In the following screenshot, you can see the headers of a typical JSF 2.2 AJAX request:

In a managed bean, you can determine the type of the request, as shown in the following code:

public void requestTypeAction() {

  FacesContext facesContext = FacesContext.getCurrentInstance();
  ExternalContext externalContext = facesContext.getExternalContext();
  Map<String, String> headers = externalContext.getRequestHeaderMap();
  logger.info(headers.toString());

  //determination...