How to check if my refurbished iPhone is original?

According to the model number it is not refurbished. Or at least it wasn't when it was sold by Apple or a retailer. This does NOT guarantee that has not been altered or 'refurbished' by an unauthorized party after the original sale.

Based on the serial number, it is an iPhone 8 Plus that is no longer in warranty.

The "M" in the Model Number would indicate that it was sold as a "new" iPhone.

You can also check to see what, if any, service coverage it has by entering its serial number here:

• Check Your Service and Support Coverage - Apple Support