How to test a service using $resource in angular

I have the following setup (forgive me if you don't use CoffeeScript)

Authentication service

class Authentication
  constructor: ($resource) ->
    @$resource = $resource

  authenticate: (user) ->
    response = @$resource('/api/v1/login', { email: user.email, password: user.password }).save()
    return response.$promise

Session controller (uses Authentication service)

class SessionController
  constructor: (Authentication, $cookieStore, $location) ->
    @Authentication = Authentication
    @$cookieStore   = $cookieStore
    @$location      = $location

  login: ->
    promise = @Authentication.authenticate(@user)
    promise.then(
      (response) =>
        # How do I test to validate the following:
        @a= true
      (response) =>
        # And how do I test to validated this:
        @a = false
    )

My question

What I can't figure out is how to test this? I would like the test to validate the value of @a after calling @Authentication.authenticate(@user)

Thank you.