Windows 8 keys are encrypted stored in the Bios installed at the factory.  There are no more COA Stickers due to piracy. (OEM System Builder Versions Do Still have COA Stickers)

Windows 8 now comes with a Genuine Microsoft Label there are NO MORE Certificate of Authenticity (COA) Labels.See the following examples of labels to look for.Color-Shifting Ink
The color shift on the label is designed to change from one color to another when viewed from different angles,making it easy to determine if it is genuine.Windows 8: magenta to greenWindows 8 Pro: blue to purpleWindows RT: turquoise to purple
Genuine Microsoft Labels
Genuine Microsoft Labels

You could try Belarc Advisor.
I don't know if this shows the key or not. (Versions Higher than 8.3c do Show the Key)Version 8.4 supports Windows 8.1
  • Operating Systems: Runs on Windows 8.1, 2012 R2, 8, 2012, 7, 2008 R2, Vista, 2008, 2003, XP, 2000, NT 4, Me, 98, and 95. Both 32-bit and 64-bit Windows are supported.
  • Browsers: Runs on Internet Explorer, Firefox, Safari, Opera, and many others.
However as you change versions by adding softpacks you should save the previous keys as they are removed and replaced with the PRO and MCE packs.  But windows will not clean install with an update pack so you need the original key in order to Install clean then do windows anytime upgrades back to PRO or MCE