iOS: How to Recognize that We Got Back from a Child UIViewController within the Parent UIViewController?

Let's say that I have 2 UIViewControllers on a stack within a UINavigationController. In the "parent" we call "[self.navigationController pushViewController:childViewController animated:YES];" upon some user action and in the "child" we call "[self.navigationController popViewControllerAnimated:YES];" upon some user action.

How can we recognize within the parent that we just got back?

Is there some "event" driven method that can recognize that this popViewControllerAnimated action was called from the child?