(iOS 9, Swift 3) If you want to find out which observers are currently registered in NotificationCenter, break and print its debug description:
(lldb) e print(NotificationCenter.default.debugDescription)
Each line of the output will contain (Notification) Name, Object, Observer, Options. Multiple calls to NotificationCenter.default.addObserver with some NSNotification.Name will result in multiple entries in this list.
NB. while this might prove useful information when debugging, I would not advise to manage observers at runtime using this output.
(source: answer based on useyourloaf)