how can i know if my battery is original

The first sign of a non-genuine battery would be a notification that would show. Unable to verify this iPhone has a genuine Apple battery. Health information not available for this battery. Learn more…

For more information on how to determine if you have a non genuine battery : https://support.apple.com/en-us/HT210323

Apple always recommends going to Apple Retails stores, Apple Authorized services providers, or sending in the device to get any repair, as this guarantee's you will receive nothing less than Genuine replacement parts.

Hope this helps ;)

Did you get a notification that your battery was not original? If NOT, then it is probably original.

Open Settings ➔ General ➔ About: Do you see a warning here that states that your battery is not original? If NOT, then your battery is probably original.

Axel F.