Google Maps V3: Check if marker is present on map?

In Google Maps V3, is there a way to check whether a marker is actually present on the map?

I have markers that vanish when clicked. I'd like some logic to check the present visibility of the marker.

For example:

var start_marker = null;
start_marker = new google.maps.Marker({ position: location, map: map, clickable: true });
google.maps.event.addListener(start_marker, 'click', function(event) {
  start_marker.setMap(null);
}); 
// ... Later in code: check whether marker is currently visible. 
console.log('Type of start_marker is now: ' + typeof(start_marker));

I was hoping this would give me a null type when the marker isn't visible, but in fact it's still an Object.

So, how else can I check whether this particular marker is visible on the map?

Thanks!