How can I know that my iPhone 12 Pro Max is genuine?

Did you buy it from a trusted seller? It's pretty hard to fake an iPhone.

Still, if you have any concern, enter your serial number in this link --> Check Your Service and Support Coverage - Apple Support

Do you have any reason to doubt that it is genuine? You can enter the serial number here→Apple - Support - Check Your Service and Support Coverage to check. Counterfeit iPhones always run Android, not iOS so that should be an easy way to tell.