Is it possible to figure out this: People who clicked on ads - which keywords have they used in searh engine? I'm not asking for high paid keywords.
You can see from which keyword they came to your site but you can see that which visitor clicked on which ads . Because this is for advertisers money security . Once bad guys find that this add is paying to much , they will eat up whole program money .
If you have a statistics program like webalizer, you could correlate your exit pages with the search queries used to find those pages. However, this is about as accurate as doing brain surgery with an axe.
You can use StatCounter or Google analytics to monitor your traffic, what keywords were used, and when. In order to narrow it down and get a general idea of what keywords were used to generate adsense income, make sure all your adsense ads have channels that you can identify by name. Compare the income generating ads to your statCouner traffic report and you will have a good idea of what is working and what is not.
Google Analytics on my site identifies with keyword a surfer used to reach my website. I have a blog and I optimize my blog post to fit this keyword traffic. That seems to help with traffic.