Do you find that SE traffic is better for adsense, then visitors from blogs, forums? is there a way to figure that out?
It is a better earner, yes. The reason being is that people have been brought to your site by a search term they are interested in. Since the ads are contextual they are probably related to that search term too, so they're are more likely to be attractive to the visitor.
Plus, your SE traffic would never be classified as "bad" traffic by adsense as the referral site (search engine in this case) as well as keyword visitor used to get to your web site are clearly visible to adsense system. SE traffic is the best traffic for adsense. Period.
Sure. Why do you think people get banned? Did you ever consider that AdSense code itself is a tracker that is counting any visitor on your site visiting adsense-enabled pages? They don't even have to click the ads for tracker to work. It is very similar to Google Analytics, but naturally collects and stores much more information than Analytics does. So, yes. Google AdSense system has a very clear idea where your traffic comes from who the people visiting your pages are, what pages they viewed and subsequently what ads they clicked.