How can I tell if a WD HDD is genuine or OEM before I buy it?

WD Internal Drives Desktop & Mobile Drives
1

I’ve bought a OEM WD HDD before and so I couldn’t register it (no warranty).

2

OEM drives are genuine … they are made by Western Digital but sold to OEM computer manufacturers which the warranty will be under.

https://support-en.wd.com/app/answers/detail/a_id/7820

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)