This will vary by device, but you should be able to:
Dial *#06# on your phone's keypad - the IMEI will display automatically
On iPhone: Go to Settings > General > About
On Android: Go to Settings > About Phone > Status > IMEI Information
Check the original packaging or the device's SIM tray
On the back of some phones or under the battery (on devices with removable batteries)
Your IMEI is a unique 15-digit number that identifies your specific device.