How to check if iphone is refurbished or original

Get the model number from Settings->General->About->Model, if it starts with an F its refurbished. If it starts with an N it was a replacement. If it starts with an M, its a normal iPhone, if it starts with a P, its a personalized (engraved) iphone.

https://www.theiphonewiki.com/wiki/Models

Read Checklist for Buying a used iphone

Where did you purchase it?