Q: How to know that my macbbok is the genuine one ?

If this is a used Mac, Apple will not guarantee the warranty the seller is providing regardless of genuine or not.

Aside from that, the Mac is associated with the original owner's Apple ID.

Find the serial number of your Mac and paste it there: https://selfsolve.apple.com/agreementWarrantyDynamic.do

How to find serial number:

Find the serial number of your Apple hardware product - Apple Support

To try and counterfeit a Mac makes no economic sense.  Usually the question is if it is a stolen MBA if the price is TOO good.

If the seller is providing a warranty,get it in writing.  That is a contract between you and him.  Apple has nothing to do with that.

Ciao.