How to find out version of Windows 7/8/10 from ISO Image?

Assuming you can mount the image, run cmd as administrator and type:

dism /Get-WimInfo /WimFile:A:\sources\boot.wim

A is your iso mountpoint.
or just extract the

boot.wim

file to somewhere and use its location in the command above.

C:\WINDOWS\system32>dism /Get-WimInfo /WimFile:D:\sources\boot.wim 

Deployment Image Servicing and Management tool
Version: 6.3.9600.16384

Details for image : D:\sources\boot.wim

Name : Microsoft Windows Setup (x64)
Description : Microsoft Windows Setup (x64)
Size : 1,264,587,270 bytes
Architecture : x64
Hal : 
Version : 6.2.9200
ServicePack Build : 16384
ServicePack Level : 0
Edition : WindowsPE
Installation : WindowsPE
ProductType : WinNT
ProductSuite :
System Root : WINDOWS
Directories : 2977
Files : 13452
Created : 2012-07-26 - 07:40:09
Modified : 2012-07-26 - 09:04:49
Languages :
        en-US (Default)

The operation completed successfully.

However newer images can have multiple indexes (AIO Installer), so you get more details by adding /index:1 or equivalent. See here for a Windows 8 example (from cmd as Administrator):

C:\WINDOWS\system32>dism /Get-WimInfo /WimFile:D:\sources\boot.wim

Deployment Image Servicing and Management tool
Version: 6.3.9600.16384

Details for image : D:\sources\boot.wim

Index : 1
Name : Microsoft Windows PE (x64)
Description : Microsoft Windows PE (x64)
Size : 1,187,717,206 bytes

Index : 2
Name : Microsoft Windows Setup (x64)
Description : Microsoft Windows Setup (x64)
Size : 1,264,587,270 bytes

The operation completed successfully.

Windows PE is the installation environment, so I guess here we really want to look at index 2 instead.

C:\WINDOWS\system32>dism /Get-WimInfo /WimFile:D:\sources\boot.wim /index:2

Deployment Image Servicing and Management tool
Version: 6.3.9600.16384

Details for image : D:\sources\boot.wim

Index : 2
Name : Microsoft Windows Setup (x64)
Description : Microsoft Windows Setup (x64)
Size : 1,264,587,270 bytes
Architecture : x64
Hal : 
Version : 6.2.9200
ServicePack Build : 16384
ServicePack Level : 0
Edition : WindowsPE
Installation : WindowsPE
ProductType : WinNT
ProductSuite :
System Root : WINDOWS
Directories : 2977
Files : 13452
Created : 2012-07-26 - 07:40:09
Modified : 2012-07-26 - 09:04:49
Languages :
        en-US (Default)

The operation completed successfully.

Sources: superuser.com