Files will be deleted from your app's document directory when the user uninstalls the app. Knowing this, all you have to do is check whether a file exists as the first thing that happens in application:didFinishLaunchingWithOptions:
. Afterwards, unconditionally create the file (even if it's just a dummy file).
If the file did not exist at time of check, you know this is the first run since the latest install. If you need to know later in the app, save the boolean result to your app delegate member.