Sorry, I thought you had figured it out.
The Webmaster Tools includes statistics on how many pages were indexed (or not), found, etc. go to Google Index > Status > Advanced, then check Total Indexed and Ever Crawled. If indeed Google is no longer indexing your site, they you should see continued increase in Ever Crawled and no increase or drop in Total Indexed. It isn't unusual for Google to trim pages from their indexes they deem unworthy. But generally speaking, the Total Indexed should increase over time.
It might be worth checking "Blocked by robots" in the Webmaster Tools > Google Index > Status > Advanced page. A sudden drop in indexing could be because Googlebot thinks your blocking it.