Open the Settings app
Tap General
Tap About
Scroll to find the IMEI number
Press and hold the IMEI to copy it
Dial *#06# on the Phone app to display the IMEI
Check the SIM tray on some iPhone models
Look on the back of the iPhone on older models
Check the original box for the IMEI label
Connect the iPhone to a computer and view device details in Finder or iTunes