How to know iPhone is Original (retail) model or refurbished

1. Go to Settings > General > About.
2. Scroll down to the Model section. You'll see a collection of numbers and letters to the right of the Model designation, but the important part is the first letter:

If the first letter is M, your phone is retail unit.
If the first letter is N, your phone is replacement unit.
If the first letter is P, your phone is personalized (engraved) unit.
If the first letter is F, your phone is refurbished unit.

You may go to Apple's Check Coverage website. You can type your iPhone's serial number into this site to see whether or not it has been activated before.

If the phone you're buying has been activated, that doesn't necessarily mean it's refurbished;

however, this process will weed out people attempting to sell a phone as New when it's actually used.

Type your phone's serial number and verification code, then click Continue.

Doing so will take you to your iPhone's diagnostics page, where you'll see something like This phone has not been activated if the phone is new.