How to detect Game Center Authentication popup

Question

I've implemented the Game Center into my game. However, one thing bothers me which is how to detect when the Game Center launches the popup to sign in. If the user is quick with their fingers and the network is a bit slow, they are able to start a game before the popup starts, which is fine, but I want to pause the game while they sign in. I thought that the UIApplicationDelegate method - (void)applicationWillResignActive:(UIApplication *)application would be called, but this is not the case.

Is there a way to detect this and thus letting me pause the game while the user attends to the popup?