How to check if this 20W USB-C adapter is original?

You may also find this support article helpful: About Apple USB power adapters - Apple Support

The bottom line? If you purchase a USB power adapter, directly from either Apple or an Apple Authorized Reseller, then it would be an original product. Otherwise, you take the chance that it is a fake.

Hello,

Hmm, looks OK to me, but with accessories it can be hard to tell. See this:

Identify counterfeit or uncertified Lightning connector accessories

Good luck.