How to test for MU via functions.php?

What is the call to determine, from a theme’s functions.php whether the site is an MU site or not?

Updated with answer:

$dir = 
 is_multisite() ? 'wp-content/blogs.dir/'.$current_site->blog_id.'/files/' 
 : 'wp-content/uploads/';

Try the is_multisite() conditional, more documentation here: http://codex.wordpress.org/Function_Reference/is_multisite

You may find the is_main_site() conditional useful as well. Just note that is_main_site() will always be true if is_multisite() is false.