The AdSense has added a Site Diagnostics tool which will help webmasters determine if pages are being blocked or not from the Mediabots that Google sends out. Here is a little demo of it:
What is the site diagnostic feature?
The Site Diagnostics page within your account provides details about whether our crawler was able to access your site in order to provide relevant ads. Our crawler sometimes encounters errors when attempting to crawl a page to determine its content. Using the site diagnostic section of your account, you can review the errors our crawler encountered for specific blocked URLs, then correct them in order for us to crawl and deliver the most relevant ads. (via)
