Testing Laravel Socialite callback

I am pretty new to testing and, in particular, mocks. I have a controller (albeit incomplete)

class RegistrationController extends Controller
{

    public function __construct(Identity $identity, User $user){
        $this->identity = $identity;
        $this->user = $user;
    }

    /**
     * Redirect the user to the social provider ie facebook/google etc
     *
     * @param string $provider
     * @return \Illuminate\Http\RedirectResponse
     */
    public function redirectToSocialProvider($provider)
    {
        // Check to see if the provider is supported, if not
        // redirect the user back to where they came from
        if(!array_key_exists($provider, Config::get('social'))) return redirect('/');

        return Socialite::driver($provider)->fields([
                'first_name',
                'last_name',
                'email',
                'gender',
                'birthday',
            ])->redirect();
    }

    /**
     * Handle the call back from social providers
     *
     * @param string $provider
     * @return \Illuminate\Http\RedirectResponse|\Illuminate\Routing\Redirector|void
     */
    public function handleSocialProviderCallback($provider)
    {
        // Check to see if the provider is supported, if not
        // abort the signup
        if(!array_key_exists($provider, Config::get('social'))) return redirect('/');

        $method = 'handle'.studly_case($provider . "Callback");

        if (method_exists($this, $method)) {
            return $this->{$method}();
        } else {
            return $this->handleMissingCallbackMethod();
        }

    }

    /**
     * Handle Facebook callback
     */
    public function handleFacebookCallback()
    {
        // Get the user information from facebook
        $providerUser = Socialite::driver('facebook')->fields([
                'first_name',
                'last_name',
                'email',
                'gender',
                'birthday'
            ])->scopes([
                'email', 'user_birthday'
            ])->user();

    }

    /**
     * Handle any missing call back methods
     */
    private function handleMissingCallbackMethod()
    {
        //
    }

}

with routes

Route::get('auth/register/{provider}', 'RegistrationController@redirectToSocialProvider');
Route::get('auth/register/{provider}/callback', ['as' => 'users.provider.callback', 'uses' => 'RegistrationController@handleSocialProviderCallback']);

How would I test the callback route to ensure that the user() method is being called on laravel socialite in the "handleFacebookCallback" method for example.