No event is generated that is capturable and there is no callback. Your app simply goes to the background.
You use the GetResumed() command to determine if you've just come back from being sent to the background. The command works fine in Android since v1076. It works in iOS since v1083 and I added the tweak for iOS for v1076.
The check needs to be done as the very first thing in the app::Loop method so that you can restore anything that needs restoring.
Cheers,
Ancient Lady
AGK Community Tester and AppGameKit Master