In case of ticket inspections by ATAC personnel, you´ll just have to show them your card or the device used for validation. The ticket inspector will simply check your card number.
If you are aboard a Cotral line (urban route only: bus, Roma-Lido or Roma-Viterbo railways) you must necessarily have downloaded the Atac Tap & Go ® application (only on Andorid devices equipped with NFC technology) and show it to the inspectors. At the moment Tap & Go ® cannot be used on Trenitalia vehicles.
Remember to travel within the validity limits of your ticket.