I have an external disk which I used to boot from, but I can't remember which version of macOS is installed. Where do I look in the directories to determine which version is installed? I am not booted into it, and I don't want to boot in yet, so can't just look in About this Mac. The System folder itself is dated 22 March 2022 so it's quite recent.
Try /System/Library/CoreServices/SystemVersion.plist
Found an easy way! Since the disk is connected to the Mac, go in to System Preferences and choose Startup Disk. Click the lock to make changes, and put the computer's password in. Then all the bootable disks will be shown, and below each one is the version of macOS installed. Find the one you want to know about and note the version, then just cancel out of changing the Startup Disk.