I've had a few pages trimmed from google's indexed number on my sitemap in webmaster tools. How can I tell which pages have been dropped?
You can find out what pages Google indexes by searching Google for "site:yoursite.com" or "site:www.yoursite.com" if you use the www prefix. You could compare this list with your sitemap to discover which pages were dropped.
There are paid tools available, like the Index Checker, that claim to find which of your pages are indexed and which aren't. (Haven't used it, so I can't offer a personal recommendation.)