Methods and apparatus are disclosed for use in an RFID system comprising a plurality of RFID devices and at least one reader which communicates with one or more of the devices. In one aspect of the invention, an identifier transmitted by a given one of the RFID devices is received by a reader or by an associated verifier via the reader. At least first and second codes are determined, by the reader or verifier, with the first code being a valid code for the identifier, and the second code being an invalid code for the identifier. The reader, or verifier via the reader, communicates with the given device to determine if the device is able to confirm that the first code is a valid code and the second code is an invalid code.