Check the Box and Phone
This may sound pretty obvious, but one of the quickest ways to determine if an iPhone is new or used is to check the physical condition of the box and the device itself. You should do this before proceeding to any other steps.
Check to see if the box is still sealed. Besides that, you’ll want to closely inspect the phone itself to make sure there aren’t any blemishes or marks. If you bought the phone from a secondary marketplace, boot it up so you can check to see if it’s a real iPhone or not.