Find the wim file located in :\sources on the install medium then run a command shell as administrator.
dism /get-wiminfo /wimFile:<Drive>:\sources\<Wimfile>
As a result you should get something like this:
dism /get-wiminfo /wimfile:Y:\sources\boot.wim
Deployment Image Servicing and Management tool Version: 6.3.9600.17031 Details for image : Y:\sources\boot.wim Index : 1 Name : Microsoft Windows Recovery Environment (x64) Description : Microsoft Windows Recovery Environment (x64) Size : 1'956'357'322 bytes The operation completed successfully.
This version number is pointing to “Microsoft Windows 8.1, 6.3.9600.17031 (IR3)”