Displaying an error when a user tries to submit an angular form that isn't completed

I'm currently working with an Angular form where the user clicks "Next Step" and the next portion of the form shows up. However, I have it so that the next step button is disabled until all fields are valid.

My question is, how can I add a class I've made to highlight the field that's not completed when the user clicks the next step button? Should I use $scope.watch or is there an easier way to root out the source of the invalid form?

if I understand the question correctly, the documentation has a good example of what you are trying to accomplish: http://docs.angularjs.org/api/ng.directive:form

there a message is displayed if the field is empty

<form name="myForm" ng-controller="Ctrl">
  userType: <input name="input" ng-model="userType" required>
  <span class="error" ng-show="myForm.input.$error.required">Required!</span>