How can I verify if the 20W ZP/A power adapter is genuine?

As already posted, there’s no definitive way (other than by purchasing a still-sealed unit from an Apple-authorized outlet) …

… however, if you connect it to — and power-up a MacBook — a genuine Apple product will display a surprising amount of AC Power Adaptor data — including the serial number — in the MacOS’ System Report.

While still NOT “definitive” (as this internal programming can also be counterfeited) it might give you a “more informed” level of confidence as to it’s “genuiness.”

there is no way to verify it apart from purchasing it from an apple store or verified apple reseller,

a fake product could still use a valid serial number