how to chekout if the iphone is brand new or refurbished (befor I buy it in the store)?

Your best assurance would be to only buy from Apple or an Apple Authorized reseller as they are the only sources who have legitimate direct access to brand new OEM iPhones. ALL Apple authorized resellers are listed on Apple's web site - https://locate.apple.com/ and if a store is not found there, then it is not an authorized reseller.

Neither Apple nor any Apple authorized reseller sells refurbished iPhones to retail customers. Refurbished devices are used for exchange service units, but they are not sold at retail.

Some carriers sell used devices at discount, but they will make it clear that those are not brand new OEM devices. In the USA at least, it is blatantly illegal to sell a used or refurbished device labeled or referred to as "new".