1. IP Address
It is the easist and must be recognized by everyone. If those clicks on your ads are originated from the same IP Address as the one used for accessing your AdSense account, your account is flagged.
2. Click Through Rate (CTR)
Normally, Click thru Rate should not excees 10%. Otherwise, Google will flag your account. For your information, normal CTR should ranges from 0.5% - 10%.
3. Physical Location
Google has good tracing software and technology.They can trace traffics origin down to the small town. So, using different computers with different IP address does not secure anything. So, don't try to click your ads in various internet cafes. That will kill you.
Most home users do not use static IP Address for Internet connection. In most cases just disconnect and reconnect will give you a new IP Address. But don't forget, Google has set cookies on your computer. They can trace these cookies and see whether they originate from the same computer.
5. Click Pattern 1
It is also suspicious when people click on their clicks and then run away immediately (hit-and-run). But normally, people will surf for a while inside your pages and then click on the ads they want.
6. Click Pattern 2
why this computer / IP address / person is so trigger-click-happy on this particular website but never click on the ads on other sites?
7. Click Pattern 3
And why is it that people accessing these sites direct (type-in URL or from bookmark) tend to be very active ad-clickers compared with those referred from search engine or other sites?
8.Other Google Services
Apart from Google Adsense, Google also provide a series of services to us. Don't just think that it is safe if you do not log in your adsense account and click on your ads. What other Google services do they provide to us? Here are some: Gmail (most poeple are using it), Google Earth, Google Calendar, Google Search, Google Toolbar, Google Talk, Google Sitemap, Google Desktop, Blogger, or even Youtube (coz Google has just recently acquired it).
9. Search Engine Ranking
Your website is not indexed on any search engine, not linked by any prominent website, but get consistently high traffic? How come people can access your website and click your ads? That will make Google to smell a rat.
10. Webpage design
How about the "Please click a link below" or "donate us by clicking the ads"? These kinds of encouragement is not in line with Google's TOS. Google can use their winning search engine, or even human eyes to check your sites from time to time.
11. Advertisers conversion rate
Ad click is one thing. But does it bring value to the advertisers? If none of the clicks on your site translate to conversion to the advertiser, you are in trouble. First the Smart-Pricing hits, then your AdSense account disabled.