How to check SN for Apple charger 20w and know if it's original?

Hello,

I don't think that you can check anything useful by using the serial number, but see this:

About Apple USB power adapters

Identify counterfeit or uncertified Lightning connector accessories

Good luck.

You possibly could check serial number on any apple accessories. But, there is nothing you can do to it in order to verify as OEM or Fake.

All you can do by check quality of accessories and its packaging.

There is no concern if you bought them from Apple Authorized Reseller.