angularjs $watch old value and new value are the same

You can use $watch instead, that seems to work. If you want to watch all the properties on the object as well (as you are doing), you need to add true as the third parameter to the watch. This sets up a deep watch.

Here is a working plunker.

JS:

app = angular.module('myApp',[]);

app.controller('MyCtrl', function($scope, $timeout){
  $scope.markers = {};
  $scope.$watch('markers', function(newValue, oldValue){
    console.log('being watched oldValue:', oldValue, 'newValue:', newValue);
  }, true);
  $timeout( function() {
    $scope.markers.foo = 1;
  }, 500);
  $timeout( function() {
    $scope.markers.bar = 2;
  }, 500);
});