How to test selectors on proxy-objects in objective-C?

How to test selectors on proxy-objects in objective-C?

Is there any way to test selectors/methods that are typically accessed
only via proxy objects?
/* Proxy which may forward
* the method to some other object */
id proxy = [UINavigationBar appearance];
/* This condition returns FALSE
* despite the fact that the method will be
* successfully executed at its destination */
if([proxy respondsToSelector:@selector(setBarTintColor:)]){
[[UINavigationBar appearance] setBarTintColor:color];
}