How to tell if a user is logged in?

If the user is logged in, I'd like to show a 'view cart' link instead of a 'sign in/register' link.

Is there a reference anywhere to global variables, or a convenient snippet I can drop in?

I'm hoping it's something like:

if ($user_logged_in) {.... or something similar

Thanks

There's an example of that already in the default tpl_header.php: The login link turns to a logout link if the customer is already logged in. And the "shopping cart" and "checkout" links appear if something is in the cart.

Thanks Dr. Byte,

I'd overlooked that as I'd customized that page a lot and ripped a lot of code out.

here it is for other people's sake (just testing a session):

<?php if ($_SESSION['customer_id']) { 
  		$content .= 'logged in content here';
  } else { 
      	$content .= 'logged out content here';
  } ?>