There are ways and factors on how Google ways detect fraud clicks that leads to banning of adsense account. These are based from my own perspective as adsense user.

1. Same IP Address- under Google terms and conditions is to avoid clicking your own ads. You can track that you have clicked your ads because of your IP address. An adsense in a site having IP address clicked by same IP address itself is no no.
2. High Click through Rate- many adsense users that got terminated have the same observation. Their click trough rate exceeds more than 10 percent. Normally it ranges from 5-10 only.
3. System of Click- where it is made. If the person search the website directly in the search engine and click the ads after appearance of the site
4. Inappropriate Click Pattern
   1. Hit the pages, click the ads then run
   2. Click ads on certain site and do not click on other site
   3. A very hot ads clicker
   4. Irrelevant Ranking- high rankings without topic getting indexed
   5. Geographical Location- only in the country of the publisher the ads was clicked. IP-country-region-city codes is always the same.