Is there a file somewhere in the System folder or in the Library that shows me which OS is installed on the disk?
You can read the SystemVersion.plist on that particular volume-
/Volumes/<volume_name>/System/Library/CoreServices/SystemVersion.plist
If there's a valid, bootable system on it, then it will be listed in System Preferences > Startup Disk, with its version number.
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2@SteveChambers - volumes are bootable, but encrypted. Maybe that's the issue here? – SPRBRN Aug 16 '20 at 15:10