We have over a thousand documents that I need to verify whether they are being shared and if so with whom. There doesn't appear to be a way to list all users granted access whether internal or external. Is there a way to do so?
Take a look at FlashPanel for Google Apps. Once you scan all your assets you might be able to filter Docs (assets) as you want.
Available in the Marketplace here.
I would strongly suggest you look at General Audit Tool for Google Apps. This is currently free (don't know about the future), and provides this and more. It can be accessed via the marketplace. It provides individual users access to discover this, as well as domain admins an overview of all docs and sites in your domain.
As of December 2019, G Suite administrators have access to the Drive Auditing log
Step 2: Understand Drive audit log data
File visibility and sharing permissions
When a user in your domain shares a file, a "User Sharing Permissions Change" event is added to the audit log.
To see files that are shared with users outside of a domain, under Visibility, select Shared Externally.