Scratch cards

There are many services which use scratch card method i.e for example there is a isp who sales internet access hours on different public spots i.e supermarkets etc.
my question is what is there mechanism of encodeing the pin code on every card thereason to ask this is the what is the possiblity chance that I purchase a card and some one has already used it without scratching i.e by guessing the serial no. series.