Once an account or card is set up for odometer readings, the cardholder will be prompted for the odometer reading when the card is swiped at the terminal.
However, please note that this might not be available at all fuel sites.
Even if the transaction is needing to be processed by a manual voucher (for example if the terminal is down), the odometer reading can still be recorded on the manual voucher.
Odometer readings are reported on your statements in the section that breaks down the spend per card.