Android
You can find one of several free apps on Google Play to look up your Android Advertising ID.
iPad/iPhone/iPod
You can find one of several free apps on the Apple AppStore to look up your IDFA. To find one, search for the terms “device identifier” within the App Store and select one of the many apps that indicates display of the IDFA.
Universal Windows Platform
You can find one of several free apps on the Windows Store to look up your Advertising ID.