Sounds simple (and maybe it is), but bear with me.
While on a support call, I asked a client what version of windows they had. XP, Vista, 7, 8, 8.1, 10...etc? They didn't know. They could have any version and each one is slightly different.
System properties is a standard method. Right click "My Computer" and go to properties. Well where is "My Computer"? Maybe it just says "Computer" or "This PC". It's in the Start Menu which looks different for each version. Maybe there is no start menu.
In windows 7, clicking the start icon loads the pop up menu. In windows 8.1 it throws up the whole start screen. You can right click to start icon and select "System" (I think).
There's a keyboard shortcut, Windows Key + Pause Break. What if they have no Windows Key? What if it's a laptop that requires the Fn key to hit "Pause Break"?
The only semi-reliable method I could think of was a process of elimination. Is there a start button? Click it, what do you see? etc.
The best I could find was websites that tell you the OS that's running. That requires an email or spelling out a URL. http://windows.microsoft.com/en-ca/windows/which-operating-system
Is there any non-technical and version independent way of determining what version of Windows a client is using?