On any phone, IMEI number could be found by typing on the keyboard *#06# and pressing the call button.
To find out IMEI of iPhone or iPad, on the screen sequentially open Settings-> Main-> About the device.
Also, IMEI number can be written on a tray for SIM-card, and in 1st generation iPhone – on the back of a metal case.
In many phones, the sticker with IMEI number can be below the battery and also on the bottom or on the side of a package.
Often, IMEI number is written on the warranty card or in a bill.