Hook onto NSNotificationCenter to see all notifications.

AppDelegate.m

        [[NSNotificationCenter defaultCenter] addObserverForName:nil object:nil queue:nil usingBlock:^(NSNotification *note) {
            NSLog(@"%@: %@", note.name, note.userInfo);
        }];

I use CF at first for that, but turns out even the modern block-based API supports capturing everything (don't do this in production!)