ZoomService allows you to register ZoomListener for receiving Zoom change events. Every time when zoom level for a specified web page has been changed, the onZoomChanged event will be fired: 

// Listen to zoom changed events
ZoomService zoomService = browser.getContext().getZoomService();
zoomService.addZoomListener(new ZoomListener() {
    public void onZoomChanged(ZoomEvent event) {
        System.out.println("event.getURL() = " + event.getURL());
        System.out.println("event.getZoomLevel() = " + event.getZoomLevel());
    }
});