How do i know when someone is logged in?

Ok..i give up searching for this solution. I need some help from you guys.

In a page template, I want to be able to show some message if user IS registered and logged in and something different if it´s not.

I´ve tried the following approach, wich uses a conditional to figure out if the user level is 0. The problem with this is that non-registered users also qualify as true for this code…

Am i doing something wrong? Is there a better approach, given the fact that I only need to know wether the user is logged in or not?

Thank you very much!

<?php
if ($user_level >= 0) {
?>

“SHOW SOMETHING FOR REGISTERED USERS HERE”

<?php
} else {
?>

“SHOW SOMETHING FOR NON-REGISTERED USERS HERE”

<?php
}
?>