code: public static boolean isPostback(final FacesContext context)
     {
        RenderKitFactory factory = (RenderKitFactory) FactoryFinder.getFactory(FactoryFinder.RENDER_KIT_FACTORY);
        RenderKit renderKit = factory.getRenderKit(context, RenderKitFactory.HTML_BASIC_RENDER_KIT);
        return renderKit.getResponseStateManager().isPostback(context);
     }
docs: Determine if the current request/FacesContext is in PostBack state