code: function() {
          var onScreen = false;
          if (this.map) {
              var screenBounds = this.map.getExtent();
              onScreen = screenBounds.containsBounds(this.bounds, true, true);
          }    
          return onScreen;
      }
docs: Method: onScreen
  
  Rreturn:
  {Boolean} Whether or not the marker is currently visible on screen.