How can I see which macOS version is installed on an external disk, without booting into it?

Question

Is there a file somewhere in the System folder or in the Library that shows me which OS is installed on the disk?

Answer 1

You can read the SystemVersion.plist on that particular volume-

/Volumes/<volume_name>/System/Library/CoreServices/SystemVersion.plist 

Answer 2

If there's a valid, bootable system on it, then it will be listed in System Preferences > Startup Disk, with its version number.