Thank you for the explanation, not that you owed me any.
As for finding the books with DRM, it depends on which format they are. If they're all epubs, running the Quality Check plugin and checking for DRM is the quickest and easiest way. If they're Amazon books or several different formats, I'd mass convert them into a format you know for certain you don't have. Not all thousand books at once, but say in batches of 50. The books with DRM won't convert. Then do a search for books lacking the format you converted into. For example,
not format:LIT if you converted to LIT. You should then see all the books that failed to convert. Tag or mark them in some way so you can find them later, then do the next batch.
After you're finished, you can bulk delete the format you used for converting. Just select all the books, right-click the remove icon and choose "Remove files of a specific format from selected books".