Is it possible to tell if a user was registered through the front end or CP?
You already answered your own question, but I thought I would post an example. In the plugin's class definition file create an init method with the following:

public function init()
{
    parent::init();

    craft()-&gt;on('users.saveUser', function(Event $event) 
    {
        $user = $event-&gt;params['user'];
        $isNewUser = $event-&gt;params['isNewUser'];

        if ($isNewUser &amp;&amp; !$currentUser) {

            // do your thing, whatever it is

        }
    });
}


This should only target 'new user registrations' by users who are 'not already logged in' (i.e. public registrations through the front-end). By the time the Users Service has raised the event, it should have already passed all the other checks, regarding whether to allowPublicRegistration, etc.