How can I denote which input fields have changed in AngularJS

If you put the input in a form with a name attribute and then give the input a name attribute, you can also access the input’s $pristine property.

<div ng-controller="MyController">
  <form name="myForm">
    <input type="text" name="first" ng-model="firstName">
    <input type="text" name="last" ng-model="lastName">
  </form>
</div>

app.controller('MyController', function($scope) {
  // Here you have access to the inputs' `$pristine` property
  console.log($scope.myForm.first.$pristine);
  console.log($scope.myForm.last.$pristine);
});

You can use $scope.myForm.$pristine to see if any fields have changed, and the $pristine property on each input’s property on the form to see if that input has changed. You can even iterate over the myForm object (non-input-field objects have keys prefixed with a $):

angular.forEach($scope.myForm, function(value, key) {
  if(key[0] == '$') return;
  console.log(key, value.$pristine)
});
// first, true
// last, false