How would one know if the SharingLink is 'Anyone', 'New and existing guests', 'Existing guests', or 'Only people in your organization'?

The most straightforward way is using the Sharing Report in SharePoint Online. You can create a CSV file of every unique file, user, permission and link on a given SharePoint site or OneDrive. This can help you understand how sharing is being used and if any files or folders are being shared with guests. You must be a site admin to run the report.

To run the report (SharePoint)

1. Open the site where you want to run the report
2. On the Settings menu, click Site usage.
3. In the Shared with external users section, click Run report.
4. Choose a location to save the report, and then click Run report.

More Information here: https://docs.microsoft.com/en-us/sharepoint/sharing-reports