For security reasons we'd like to see all documents in Google Drive which are shared with people outside the organization (Google Apps users). Is this possible? I know you can search for "to:[email]" but this requires knowing the exact email, we want "to:[non apps user]".
Google Apps does not natively support reporting on who has access to what in Google Drive. The report you are looking for is possible through a Google Apps script.
As you probably know, from the
Reports section of your Google Apps admin panel, you will be able to tell how many files are externally visible.
If you are on the Google Apps Unlimited or Google Apps for Education license, you will have access to a Drive audit log, from which you may be able to generate a report based on sharing events. This could be cumbersome.
A free option based on Stefan Lasiewski's suggestion below is to try whohasaccess.com. The app requires
View and manage the files in your Google Drive permissions, among other simpler permissions.