Drupal - How know if user is visitor or logged member?

For the sake of more robust and descriptive code, you can make use of the handy User::isAnonymous() method, e.g.

if (\Drupal::currentUser()->isAnonymous()) {
  // Anonymous user...
}

  $current= \Drupal::currentUser();
  if (!$current->id()) {
    // is visitor
  }
  else {
    // is logged
  }

isAnonymous() holds the danger of not working when a site is saving data (for example in forms) in PrivateTempStore. Therefore, i prefer the following:

if (\Drupal::currentUser()->isAuthenticated()) {
  // This user is logged in.
} else {
  // This user is anonymous.
}