This is simple math and it depends on how accurate you want how much trouble it takes to get the information you seek. First, add up all the unique visitors from a raw log file report (AWStats or whatever) and compare to the same number reported by AdSense. Won't be exactly accurate because you can't be sure some of the visitors aren't bots.
Adblock visitors = raw log visitors - AdSense stats
If you want to get a more accurate count and eliminate most of the bots, install a bot blocking script (which you should do anyway) and then subtract all the junk the bot blocking script removed.
I was actually planning to build a service that does the measuring and converting. Though after your reply I'm having some doubts because it seems that only a very small amount of websites suffer from decreased revenue because of adblock users.