How will i know the battery am replacing is original Apple part

Apple doesn't sell nor distribute spare parts to anyone, except in the USA (as per this program:--> Self Service Repair - Apple support) or except to their Stores or Authorized Service Providers around the world.

Regards

Giulio

If you have it replaced at an Apple store or authorized Apple repair shop, it will be genuine. If it replaced anywhere else, including you replacing it, it will not be genuine.

Take your device to an Apple Retail Store or Apple Authorized Service Provider, or contact Apple Support: