Open the Settings app
Tap General
Tap About
Find the IMEI number
Dial *#06# on the Phone app
Check the IMEI on the SIM tray if your iPhone model supports it
Check the IMEI on the back of the iPhone if it is an older model
Look on the original iPhone box label
Check your Apple ID device list at appleid.apple.com
Check the IMEI in Finder on a Mac or iTunes on a Windows PC when the iPhone is connected