How i can know that’s my adaptor original?

Did you purchase it directly from Apple or an Apple Authorized Reseller? If so, then chances are good that it is a genuine Apple product. Otherwise, I would suspect that it is not.

Ref: About Apple USB power adapters - Apple Support

Unless you are buying from shady stores or people on the street, there's no reason to think its counterfeit.

If its a known store that sells Apple devices you can verify its an Authorized reseller here: https://locate.apple.com

Any reason to doubt its legitimacy?

The link I provided you is all that Apple offers on this subject ... which isn't much. One thing I would check would be that the label printed on the adapter is clearly imprinted and centered properly. You will just have to go on faith that this authorized reseller did not sell you a knockoff. If you have any doubts, return it and purchase it directly from Apple.

If it's an Authorized reseller, then it's unlikely they are passing off knockoffs. Is the store known for selling fake items?

Why do you doubt what the store sells? Are fake chargers sold commonly where you are? From Authorized resellers?

There is no way unfortunately to conclusively tell wether the adapter is real or not. Check the markings and make sure they match with the charges in Tesserax link is about as much as you can do to verify.

If you live in a country that uses other types of plugs, like European plugs or U.K plugs tell us which one so we can point you to the correct link for your country or region.