Under construction page and some 404 error page return code 200. Not accurate this way but your idea is not bad. A simple search on google doesn't return any mass auto header check tool.
I've messed with something similar in the past - And have to say it's nearly impossible without physically visiting the sites (domains) yourself. Everything I wrote or came up with had flaws ... Parking companies using different headers / People parking sites with many pages still crawled in engines / etc ...
Thanks for the replies guys.
I think you're right mark - it's just a case of visiting the sites - makes prospecting just that little bit more challenging.