There is a free utility called ShowKeyPlus, which is in the Microsoft Store. This will show you your product key. It will show you the original Win 7/8 key you used to upgrade if you went that route as well.
Once installed via the store you can dig around and locate the ShowKeyPlus.exe file in the hidden folder for Appx apps. It will run on its own as a standalone utility. I put it on the desktop of my golden image so I can easily check.
Of course you want to run ShowKeyPlus BEFORE you image, and save the key to add to the Fog database when you register the host (you must add manually).