How can I see which documents have been shared using New Collaborate?

The Client - Collaborate tab shows a complete history of which documents were shared with clients and other recipients, both Collaborate and New Collaborate.

To see which files have already been shared with clients on New Collaborate, you'll need access to both the New Collaborate site, and the OneDrive folders used with the previous version of Collaborate.

1. Open the New Collaborate Site on SharePoint.
   
   
2. Click the Documents menu.
   Note: By default, there will be a Documents link in the menu when the New Collaborate site is created. If you can't see the link, please refer to your OneDrive Admin who created and configured the New Collaborate site.
   
   
3. Select the appropriate Client Share Folder
   
   
   
   
4. Open the various folders to compare the files in your Client Folder on your New Collaborate Site with the previous OneDrive Client Folder used in Collaborate; this will provide you with a list of which files have already been shared, and which files still need to be shared with the client

Refer to Sharing Documents with Clients and External Users.