Been thinking about this for a while. Hacking is not only a risk, it's a reality in many cases. Some hackers delete your info, redirect traffic to other places or they just put a custom message regarding their abilities or ideology, etc.
But remembering my old studies there was a book stating that the worst hackers just modify your info for fun or to get something out of it. So in the case of hackers modifying your website inserting their adsense code (or just a random one) you will be complaining on the Adsense stuck and etc observations forum. But will you really notice the hack? yeah sure via page views perhaps.
Lots of webmasters here handle several websites, small or large networks with hundreds or perhaps thousands of pages. Some webmasters use a CMS, be it third party or custom made. Some have hand made html pages.
Anyway there are two scenarios:
#1. You must check your adsense publisher locally, on your server. Be it on each of your html pages, php includes, shtml includes, etc, or your database. What if your CMS uses some sort of cache? you have to check your cache files if the cms works that way, or the database if the cache is stored there.
#2. You have to check your website via web tool.In this case some sort of crawler will check each and every single one of your pages verifying your Adsense Publisher against the one you provide for the checking. This could be a lot easier in many ways. There might be cases where hackers insert random scripts showing random publisher codes, like yours and theirs, who knows.
Is there such service around?If I remember correctly there were cases of ISP inserting ads on some websites hosted with them, it sucks right?