So disable all background services and don't access anything that uses data on your phone and see what the carrier bills you? Unless you believe that Apple some

So disable all background services and don't access anything that uses data on your phone and see what the carrier bills you? Unless you believe that Apple somehow has secret arrangements with every mobile operator... in which case you could simply get a MiFi and tether all your smartphone traffic through that, problem solved. In the end, if you really believe something malicious is happening in this regard, you can either prove (if said data is actually transferred over WiFi) or prevent (if it's only transferred over mobile data) it, rendering mere speculation about this issue useless.