It seems strange to me that Google haven’t implemented any way to produce a report of who has access to your shared files and folders in Google Drive. If you have more than a handful of files and folders, it quickly becomes tedious to check the permissions on each file individually. This sounds like a repetitive chore, which is an ideal task for a computer to do.
Check out WhoHasAccess: https://www.whohasaccess.com
This web app will, with your permission, scan your entire Google Drive (it can take a while if you have a large number of folders) and show you a report of who has been granted access, and what they have been granted access to. If you own, or manage any of these files and folders, you can revoke their access quickly and easily.
The service is free and they claim that they “care about data protection: After the scan is completed, they’ll delete all collected data and remove their access permissions within 24 hours. They will not read any of your files’ content at any time.”