While it's not exactly what you're asking about, may be $pristine is what you want? It's a flag on the form indicating whether the form has ever been edited. If someone typed and then cleared out a field, $pristine would still be false, however.
<form name="myform" ng-submit="doSubmit()" ng-controller="FormController">
<input ng-model="firstName" name="firstName" />
</form>
Then in your controller
.controller('FormController', function($scope){
$scope.doSubmit = function(){
if($scope.myform.$pristine){}
}
})
Alternatively, you can set all your fields to required="true" and use the $valid flag on the form in the same way as described above.