In a multisite setup, you probably don’t want to turn on debugging for all your sites. Rather, just one or two.
Take a look at the $_SERVER global array – then go with conditional if/else.
For a single site, maybe:
For a couple sites, perhaps
if ($_SERVER['SERVER_NAME']== 'foo.jamesfishwick.com' || 'baz.jamesfishwick.com' ) define('WP_DEBUG', true); else define('WP_DEBUG', false);
$_SERVER['SERVER_NAME'] is based on your web servers’ config file (Apache2 in my case), and varies depending on a few directives: (1) VirtualHost, (2) ServerName, (3) UseCanonicalName, etc.
$_SERVER['HTTP_HOST'] is based on the request from the client, i.e. what is in the address bar.