Still thinking to cheat AdSense? Stop that. It will never get you anywhere. You might pull it off with smaller ads network, but definitely not with Google. Here are some detection methods they might use. At the very least, they have the resources to do so.
IP Address
If the AdSense click is originated from the same IP Address as the one used for accessing your AdSense account, your account is flagged.
Cookies
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.
Other Google Services
Thinking that you are safe just because you do not access your AdSense account? Think again. This time, consider these: GMail, Google Earth, Google Calendar, Google Search, Google Toolbar, Google Talk, Google Sitemap, Google Desktop, Blogger, and so on, and so on. With the wide range of services they provide, Google can trace the originator of most (or probably almost all) clicks.
Click Pattern 1
Oh, why this computer / IP address / person is so trigger-click-happy on this particular website but never click on the ads on other sites?
Click Pattern 2
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?
Click Pattern 3
And why the ad-clickers like to hit and run, compared with non ad-clickers that surf a few pages before leaving?
Click-Through-Rate (CTR)
Your CTR may range from 0.5% to 10%, but if it exceeds a certain point (probably around 10%), you are flagged.
Geo-Location
Used Urchin (Google Analytics) before? Then you should know that Google can trace traffics origin down to the small town. Different IP doesn't mean much. Unless you site is really targetted to one small geo-point, a high number of clicks from nearby location will get you banned quickly.
Hardware address?
MAC address of the LAN card, modem, and router works almost like a fingerprint. I'm not sure if Google can track this, but probably they do. They have rocket scientist, remember?
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.
Search Engine Ranking
Your website is not indexed on any search engine, not linked by any prominent website, but get consistently high traffic? That sounds like something is in play. Regardless of whether it is an adware-embedded software, spam, trojan clickbot, or intentionally installed click-exchange network, it doesn't sound right.
Webpage design
How about the "click here" or "support us"? Google has the best search engine in the world. Is it really that hard to find those words?
Combo
Each of these detection methods might seem rather weak. But combine them together, and not many click-fraud can pass-through these filters. Even the smartest clickbot will have a hard time.