I noticed recently that when I re-booked an OBB cruise that was cancelled, the reservation number actually changed from the original. I wondered if this was because the original OBB had an original expiration date that was earlier than the new cruise I re-booked to. In any event, another way I've verified if OBB was applied was through the Reservation Confirmation email... underneath the payment summary, there's a line "The following apply to the reservation... * Onboard Offer"