So you recently had to take your iPhone in for an appointment with the geniuses at your local Apple store and you ended up leaving with a replacement phone. Now, you're left wondering whether you got a new iPhone 4 as a replacement, or a refurbished phone.
Truth be told, there will be little way to tell. Per your iPhone warranty, Apple has the right to replace your defective iPhone with a new or refurbished unit. Replacements come in an unmarked box, whether new stock or refurbished stock, and serial numbers/etc are assigned as new.