How can I tap into onUserDelete?
From the documentation on events, they follow this pattern:

craft()-&gt;on('component.eventName', function(Event $event) {
    // ...
});


So in your example, it'd be something like this and you'd want to add it to your plugin's init() method in the main plugin class.

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

    craft()-&gt;on('users.onDeleteUser', function(Event $event) {
        $user = $event-&gt;params['user'];
        $userToTransferContentTo = $event-&gt;params['transferContentTo']
    });
}


The full list of available events and the parameters they have can be found here.