M, F, N, or 3A? What the first letter of your iPhone’s model number actually means

An iPhone may look perfect on the outside, but how do you know if it is new, refurbished, or a replacement unit?
HowToGeek on MSN

How to Check if Your iPhone Was Refurbished

Find your iPhone's model number in the Settings app. If it starts with an F, it was refurbished by Apple or a carrier.