Detect if user is a guest

There is this:

<?php
global $user_login;
if( $user_login ) {
echo 'user logged in';
} else {
echo 'user not logged in';
}
?>

This shoutbox have that functionality too

Or these:
https://faked.org/blog/wp-onlinecounter/

or WPuserOnline:
https://lesterchan.net/portfolio/programming.php