As long as windows 8.1 is installed you will be able to upgrade for free. The key is embedded in the motherboard and is not able to be retrieved by any program and is not available from Dell. You should make all the Recovery disks and system image to preserve the Recovery Partition to reinstall windows in case the hard drive fails. Windows has the recovery options you need. Buy a couple of thumb drives for making bootable recovery drives (16gb or bigger) and an external hard drive or a second separate hard drive for the system image and File History backups.Open Control Panel and search for Recovery to access the tools.  
I recently bought an X51 R2 from the outlet store and it's been great but I'm thinking ahead. The PC came without any install media, which was as advertised and I was aware. So I'm going to create recovery media blah blah blah, but there's an offer to allow any Windows 8.1 (or 7) OS user upgrade to Windows 10 for free with a 1 year window from when the new OS is released. Anyone know if that offer will still be open to me? There's a service tag on the PC, but no Windows license information. Can I use the tag to find my key? I know I can use something like Belarc to dig it out of the registry but I'm still wondering if my current key is even upgradeable. No biggy either way, just curious. 
Windows 10 upgrade for Outlet PC