How can I find out if my phone company sold me a new iPhone or used

Did they sell it to you or is it on contract. If the latter, and the phone is the model you wanted, if it had been previously used you have no grounds for concern. They should have advised you but it is not obligatory.

Did you get the usual 1-year warranty with the phone?

You could try calling Apple Support. Have the phone's serial number ready. Contact - Official Apple Support