Look up These Key Metrics
If the iPhone itself is in good shape, then you can move on to checking to see if it was used by the seller. While the physical condition of the iPhone and its packing are important, there are other metrics you can check on the device itself that can reveal whether it’s been used.
First off, go to Settings > Battery > Battery Health. A brand new iPhone should always have 100% battery health. If it doesn’t, it’s probably used or has a defective battery.
You can also go to Settings > Cellular and scroll down all the way. Check to see if there’s a number next to Lifetime under the Call Time subheading.