Mocking classes that created with app()->make

I have this code in my __construrct:

public function __construct(Guard $auth)
{
    $this->auth = $auth;
    $this->dbUserService = app()->make('DBUserService');
}

Now, when i'm unit testing i know that i can mock Guard and pass it's mock to $auth, but how can i mock dbUserService ? it's instantiated trough the IoC container.