You can find this by looking in your settings or by looking at your device.
Look in settings
Go to Settings > General > About. To the right of Model, you'll see the part number. To see the model number, tap the part number.
Look on your device
On an iPhone 8 or later
To find the model number, remove the SIM tray and look in the SIM tray slot. Make sure that you're in a place with bright light overhead. The model number is on the upper side—the side with the display.