If you boot into Windows installation media (say USB stick), and press [SHIFT] + [F10] to get the command prompt, you could query which drive letter corresponds to the original OS drive in question, and then get the version info of a program like
ntoskrnl.exe. For example to explore the installed disks and related drive letters,
$ wmic diskdrive get model,name,serialnumber
$ wmic logicaldisk get description,deviceid,volumename
$ wmic datafile where name="<DRIVE LETTER>:\\Windows\\System32\\ntoskrnl.exe" get Version /value
Or using the method from @beyondtime, which is less typing but omits the decimal part of the build number
$ type "<DRIVE LETTER>:\ProgramData\Microsoft\Diagnosis\osver.txt"
So here the OS installed on the indicated drive letter is Windows 10, Build 15063.413. A google search shows this to be Windows 10 version 1703 | Redstone 2 | Creators Update.
As a bonus, if you want to see the last time that OS was booted, you can do
$ dir /ah /tw <DRIVE LETTER>:\pagefile.sys
01/31/2017 03:04 PM ...... pagefile.sys
You can probably determine when the OS was originally installed by reviewing the creation time of the hidden files and folders at the root of the drive:
$ dir /ah /tc "<DRIVE LETTER>:\"
And to shutdown when you're finished:
$ wpeutil shutdown