Methods and programs to learn the key of the operating system, which is installed on the computer, many, but within the framework of this article I will consider only one: a proven, working and free.
Getting information about the keys to the installed Microsoft products using a free program PRODUKEY
In order to see the keys to the installed Windows 8.1 operating system, 8.1 and previous versions, you can use the PRODUKEY program, which can be downloaded for free from the developer site https://www.nirsoft.net/utils/product_cd_key_viewer.html
The program does not require installation. Just run it and it will display the keys of all Microsoft software installed software on your computer - Windows, Office, and maybe some more.
The short instruction turned out, but I just don't know what else add here. I think it will be quite enough.