iPhone - testing if a notification exists
If you mean "testing for whether you've already registered as an observer for the notification", I don't think there's an easy way apart from posting the notification and seeing you get a callback (with possibly disastrous effects).
If there's a danger of a double-add, I usually use [[NSNotificationCenter defaultCenter] removeObserver:self name:foo object:bar]
before the add.
Registering for notifications does not happen on a queue.