Google see from where the clicker came on to the site if its an invalid click it must be by the publisher or his friend the publisher comes in trouble when he sees the clicker has come direct to the site I.E not by search engine neither by any reffered site so it must be like publisher has given him the site address and the time limit of the clicked site also matters its usually the faulter just clicks on the ads and closes the page google can detect those as invaild clicks