Is it possible to tell if a user was registered through the front end or CP? I would like to be able to perform actions on those users that register through the front end and skip ones who are created in the CP... is there are way of checking or a parameter accessible in the onBeforeSaveUser or onSaveUser events?

I assume there must be something hidden in there as users registered on the front end can be assigned to a certain user group which isn't applied to a CP registration.
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.