How can I detect UIStatusBar hide and show?

You can observe the statusBarHidden property of the shared UIApplication instance.

Simple example:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    // Do something here...
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    [[UIApplication sharedApplication] addObserver:self forKeyPath:@"statusBarHidden" options:NSKeyValueObservingOptionNew context:NULL];
    [[UIApplication sharedApplication] setStatusBarHidden:YES]; // Will notify the observer about the change
}