AngularJs unit test - Check if "Init" function was called

I'm using jasmine as testframework and I've the following Controller I want to test. And I allways have a Init() function where I place my initialization calls for this Controller.

Now I want to test if the Init function was called when the controller was initialized.

function UnitTestsCtrl() {
   var that = this;
   this.Init();
}

UnitTestsCtrl.prototype.Init = function() {
    var that = this;
    //Some more Stuff
}

angular.module("unitTestsCtrl", [])
       .controller("unitTestsCtrl", UnitTestsCtrl);

But I was not able to check if the Init function was called on controller creation. I know my example doesn't work because the spy is set on the Init function after creation.

describe('Tests Controller: "UnitTestsCtrl"', function() {
var ctrl;

   beforeEach(function() {
         module('app.main');
         inject(function ($controller) {
            ctrl = $controller('unitTestsCtrl', {});
         });
   });

   it('Service Call executed: InitUnitTestSearchModel', function () {
       //thats not working
       spyOn(ctrl, 'Init');
       expect(ctrl.Init).toHaveBeenCalled();
   });
});