How can I verify if the MQKJ3ZA/A 60w power charge cable 1m is genuine?

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

Folks seem to keep posting images and/or serial numbers of supposed Apple devices/items asking us to determine if they are fake or original products. As such, there is no way anyone here can determine that from an image or serial number. You, yourself, can determine if the serial number is one that Apple recognizes here:

• Check Your Service and Support Coverage - Apple Support

Note that not all Apple products have searchable serial numbers, so even if it doesn't show up here, it may still be possible that it is an original product.

The bottom line? If you purchased this item directly from Apple or an Authorized Apple Reseller, then it should be an original product; if not, then chances are that it is a fake.