A direct answer to this question is- NO BODY KNOWS. Google hasn’t revealed this to anyone. Only the smart guys working in the google adsense program know about this. And this is totally right coz once its released , every other ad program will copy this and also users might try to find other ways to bypass their technology in place.
But then there are many common ways by which we can find out whether the ad clicks are fraudulent. You don’t need to be a computer expert to understand this. Here’s a small list by which google could find out whether the clicks are fradulent or not –
1. Why should'nt I click my own ads on my own computer? Its simple every computer has its own IP address( like your home address, its unique for every computer), so if you click your own ads google will find out. Now you might think you’re smart and go to internet cafes and click on your own ads, but sorry my friend google will still catch you. They also track the number of times an add has been clicked from some other address, if the time gap is small they will easily find out , and you’ll get banned.
2.Duration of Visitor - Google knows how long your visitor was in your website. If an individual goes to your site and clicks your ads even if the user stayed only for a few seconds in the site, can also result in ban.
3. Low Traffic - If you have low traffic, and you got clicks, it can also result in your account getting banned. For example, if you have only 20 visitors a day and you got 10 or more than clicks, thats also very obvious – goolge will keep u in the watch list. If it recurs everyday you will surely get banned.
4.Your Website Ranking: For example you just built a new website which has not been indexed by google, but then you get a lot of clicks, how the hell is that possible??? Googles easily could find this out and you might get banned.
5.Value Of Click: You might get banned for getting only clicks on your adds and no product being sold. So even if its not your fault , you might just get banned>sad right.:(((
There are softwares in place and an army of Google PhD’s working round the clock developing programs/algorithms just so that they can find out fradulent clicks.
Not only that even if a google staff comes across anything which does not go by the terms and condtions, you will get banned.
These are ways by which google could find out of fradulent clicks. But who knows, they might have many other sophisticated techniques which we got know idea off.
So the final conclusion- do not click your adds or even tell your friends to click your adds every few days or so, the best way in my opinion is to focus on your content. Add good content which might get visitors back to your site and thereby increasing your traffic which will obviously lead to an increase in the clicks and which in turn will lead to money money money:))