How can I know if my iPhone is opened?

If the Model number starts with N, it means it was a replacement iPhone.

That is, the iPhone was given as a replacement for one that has an unrepairable issue.

It can be new or refurbished at Apple's discretion. Refurbished iPhones are indistinguishable from new ones and go through a much more rigorous verification process.

The Serial number does not in any way determine wether its Refurbished, Replaced or retail new.

A model number that starts with an "F" is a refurbished iPhone. This would have been done by Apple. As it being refurbished, there's no way anyone here (we are not Apple employees) could tell you what Apple did to get this particular phone to the refurbished state.

Phones refurbished by Apple are indistinguishable from new. There's no reason to worry about it.