How can I know if an iPhone has been refurbished?

Because the model number starts with an "M". If the phone was refurbished, the model number would not start with an "M".

If the phone was refurbished by Apple, the model number would start with an "N". If the phone was refurbished by a 3rd party, the model number would start with an "F".

This was a new phone when it was sold for the first time.