How can I check my iPhone warranty without the SN or the original box?

Since an iPhone 14 is still covered by warranty (it's 1 year from the date of purchase), if you can't check that info in Settings>General>About>Serial Number, I suggest that you contact Apple Support directly, using this link:--> Contact Apple for support and service

Regards

Giulio

You can always obtain your serial number from the “Devices” section of your AppleID at https://appleid.apple.com/.