I am not sure how effective are the methods to detect whether adblock is enabled or not.Most popular method seems to be checking the size of the ad content.There are other methods being discussed here http://stackoverflow.com/questions/4869154/how-to-detect-adb...

In the article itself there is reference to anti-adblock killer.If anyone installs an adblocker plugin,they can probably install an anti-adblocker killer too.