If the laptop came with any variety of Windows 8, the product key is normally written in the BIOS. This means that you do not need to enter it, you can just activate (or reactivate) the operating system when prompted.
If you are asking this because you want to reinstall Windows or do a clean install onto a new hard drive, the Windows 8 installer will not ask you for a product key if it can find it in the system BIOS.
If you still want to look it up, I recommend using a tool like Magical Jelly Bean. I normally download it from a third party site, so I can get the standalone version (it does not need to be installed). Here is where I normally get it from: