Revenue Inspectors can tell whether a card is registered but mine isn't. I've seen Revenue Inspectors in action in the past. They confiscate the card, issue a ticket for the remainder of the journey, and take the name, address and date of birth of the passenger which they check. They then issue either a penalty fine or a notice to prosecute and advise the passenger of their right to appeal. The inspector who i spoke to, asked me to write my details in her notebook (rather than taking the details herself) she then disappeared to the lower deck for a couple of minutes, presumably to check my details, before giving me the card back!