If we knew the answer to this question people would be ripping off Google AdSense left and right I'm sure there are many different methods to detect them.
Yep. No one knows 100%, but I'm sure they use tons of variables. That's why I love it when people think proxy clicking will get them a new mansion.
Well they have a system in space and the most important variable is ip address. If u click ur ads from a different ip address just once or twice they will never catch
Google knows about every proxy service available out there. Try to cheat them and you will see what happens. Gunter