I’ve bought a OEM WD HDD before and so I couldn’t register it (no warranty).
OEM drives are genuine … they are made by Western Digital but sold to OEM computer manufacturers which the warranty will be under.
there’s no way to tell (via model/serial number) before buying if it’s Retail or OEM
If you’re buying from a Retail Store … then they should be Retail drives with WD Warranty.
If you’re buying from Computer Parts store … Ask before buying. (that’s about all you can do)