// If no click event listener, then attach the listener
google.maps.event.addListener(marker, 'click', function() {
  infowindow.open(map, marker);
});

// If no click event listener, then attach the listener
if (! marker._isClickEventBound) {
  google.maps.event.addListener(marker, 'click', function() {
    infowindow.open(map, marker);
    marker._isClickEventBound = true;
  });
}

google.maps.event.hasListeners(marker,'click')

google.maps.event.hasListeners(map,'idle')

google.maps.event.clearListeners(map, 'click');