invalids from what? you mean adsense?? this one might help: http://forums.digitalpoint.com/forumdisplay.php?f=27
As above said only Google knows the answer. I will give my own assumption based on possible ways. 1. Local IP and Geo-location - This is a common way to handle invalid clicks. If some clicks are coming from the same IP and the same local address for a period of time. Then it would be easily identical for invalid clicks 2. Machine name - This is still a myth if Google may filtered based on PC machine name. It just a possible way. 3. Checking through Google Services - This is a highly possible for Google. They can track your activities based on your logged in with their services, including Gmail, Earth, Adword, Adsense, etc.