New iPhone

Why wouldn't it be new? Why do you doubt its new? Are you not buying devices from trusted sources such as Apple Stores or Authorized resellers?

If you suspect it may not be new, without opening the box, there's no way to know what's inside exactly.

You can verify the model number on the box, but if you are having doubts about the device, then there's no reason to believe the model number on the box will match what's inside. If the model number on the box starts with an M, then at least the box was for a new device. Whatever is inside may be something else entirely. So if your having trust issues, the box is not the best thing to trust.

In other words unless you trust the place you bought the device from, the only way to know what you got is to open the box.

Buy it directly from Apple or an authorized retailer. If you buy it from anyone else, it is not, by definition, "new".

Buy it from Apple.