How to know if an iOS app does not access the internet via Wi-Fi or Cellular?
iOS 12. 
There's Wi-Fi, Bluetooth and USB-Tethering that you cannot directly toggle if you're using any of those interfaces for any purpose. Only way to stop them is to turn off the connection path itself. For Wi-Fi and Bluetooth, they should be turned off globally in Settings. For USB, detaching the cable is the way to go.