Blind yourself and test repeatedly. Either setup a script to turn data on and off, or get someone to do it for you. Then have them give you the phone without in

Blind yourself and test repeatedly. Either setup a script to turn data on and off, or get someone to do it for you. Then have them give you the phone without interacting with you. Repeat several times. If you can reliably determine when a phone is using data, that's probably something to look into.