How can I find the product key that was used to activate Windows 8?
For Win8 OEM devices there is no key on a sticker. The key is embedded to the BIOS.
The Windows 8 Product Key Viewer should work and read the correct produkt key.
// Edit: R-W Everything also shows the key under ACPI->MSDM
You can try Magical Jellybean; just tested and although it doesn't mention windows 8 it shows my key.