iOS: How do I know if a property is KVO-compliant?

In the Key-Value Observing Programming Guide, the section Registering for Key-Value Observing says "Typically properties in Apple-supplied frameworks are only KVO-compliant if they are documented as such." But, I haven't found any properties in the documentation that are documented as KVO-compliant. Would you please point me to some?

Specifically, I would like to know if the @property(nonatomic,retain) UIViewController *rootViewController of UIWindow is KVO-compliant. The reason is that I'm adding the rootViewController property to UIWindow for iOS < 4 and want to know if I should make it KVO-compliant.

@interface UIWindow (Additions)

#if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_4_0
@property (nonatomic, retain) UIViewController *rootViewController;
#endif;

@end

@implementation UIWindow (Additions)

#if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_4_0
@dynamic rootViewController;

- (void)setRootViewController:(UIViewController *)newRootViewController {
    if (newRootViewController != _rootViewController) {
        // Remove old views before adding the new one.
        for (UIView *subview in [self subviews]) {
            [subview removeFromSuperview];
        }
        [_rootViewController release];
        _rootViewController = newRootViewController;
        [_rootViewController retain];
        [self addSubview:_rootViewController.view];
    }
}
#endif

@end