-(void) viewWillDisappear:(BOOL)animated {
    if ([self.navigationController.viewControllers indexOfObject:self]==NSNotFound) {
       // back button was pressed.  We know this is true because self is no longer
       // in the navigation stack.  
    }
    [super viewWillDisappear:animated];
}

http://www.comanswer.com/question/setting-action-for-back-button-in-navigation-controller

Code taken from comment by 'William Jockusch' in the URL.

Detect back button touch on UINavigationController

iphone

iPhone