how can i check if my phone is brand new?

Check Your Service and Support Coverage - Apple Support

Find the serial number of your iPhone as follows:

Settings > General > About

Assuming that you bought from an authorized Apple dealer, if the Purchase Date of the phone agrees with your sales receipt, that is a good sign.

Next check......Settings > General > About

Make sure that the Model Number begins with an "M"

If you did to purchase it from an authorized retailer, it is not "new".