How can I check the marker is or isn’t in the bounds using google maps v3

The LatLngBounds object comes with a contains() method which takes a LatLng point and returns true if the point happens to be within the bounds, or false if outside.

Therefore, what about something like the following?

function check_is_in_or_out(marker){
  return map.getBounds().contains(marker.getPosition());
}