On Android devices, you can find your GAID(Advertising ID) in your device settings. Navigate to Settings, click Google, and then Ads:
You can also find the GAID programmatically. Learn how to get a user-resettable GAID.
Currently, Apple does not surface the IDFA by default on iOS devices. However, it can be retrieved using third-party apps("MyIDFA"). Check the App Store for apps that can retrieve your advertising ID, or find your IDFA programmatically.