Hello. So my friend has a laptop that came with Windows 7 preinstalled and was upgraded to Windows 8 Pro. I kinda doubt that he paid for Windows 8 (this was before 8.1) and the fact that it's Pro is a red flag for me. But it seems to be legit - I see no cracks or loaders installed, no KMS stuff, key is from the retail channel, nothing shows up when I google the key. It seems legit to me, but just to make sure he doesn't run into any trouble when reinstalling, I want to know how to be 100% sure that a Windows product key is 100% legit.