CakePHP 3: How to properly check if a user is logged in

I think the best way is just:

if ($this->Auth->user()) {...}

You can do this using session() helper.

$loggeduser = $this->request->session()->read('Auth.User');
if(!$loggeduser) {
    $userID = $loggeduser['id'];
    $firstName = $loggeduser['first_name'];
}