How do you test methods with no return value & how do you mock a collection of models?

Can anyone tell me how I can test the notifySubscribers method this class?

If it doesnt return anything how do I assert anything? Also it takes a collection of User models as a parameter, do I need to add a mock collection? or a collection or mock user models? How do I add state to these models?

Thanks.

namespace App\CoreServices;
class Notifier {
 /**
 * Broadcaster object. Does the actual pushing out
 *
 * @var \App\CoreServices\Broadcaster
 **/

private $broadcaster;

/**
* Notification Model
*
* @var \Models\Notification
**/
private $notification;


/**
 * Instanciate the notification and the broadcaster model
 *
 * @param \Models\Notification $notification
 * @param \App\CoreServices\Broadcaster $broadcaster
 * @return void
 **/

public function __construct(\App\CoreServices\Broadcaster $broadcaster, \Groopeze\Models\Notification $notification)
{
  $this->broadcaster  = $broadcaster;
  $this->notification = $notification;
}


/**
 * Notifies each of the subscribed users
 *
 * @param array $subscribers
 * @param array $message
 * @param string $eventName
 * @return void
 **/

public function notifySubscribers(array $message, \Illuminate\Support\Collection $eventSubscribers, $eventName) {        
  foreach($eventSubscribers as $i => $user) {
    $this->notification->message = $message["message"];
    $user->notifications()->save($this->notification);
    $this->broadcast($user, $eventName, $message);
  }
}


/**
 * Calls the broadcaster to push out the message
 *
 * @param User $user
 * @return void
 **/

private function broadcast(\Groopeze\Models\User $user, $eventName, array $message) {
  $setting      = $user->settings->where("event", $eventName)->all();
  $notifyMethod = $setting[0]->personalSettings[0]->setting_value;
  $message["message"] = $message["message"];
  $this->broadcaster->$notifyMethod($user, $eventName, $message);      
}

}