Dial *#06# on the phone keypad
Check the IMEI in Settings > About phone > Status
Check the IMEI on the SIM tray
Check the IMEI on the back of the phone
Check the IMEI on the original box or label
Check the IMEI in the phone bill or purchase receipt
Check the IMEI in your Google account device details
Check the IMEI in your Apple ID device list or Finder/iTunes backup info
Check the IMEI in the carrier account or mobile app
Check the IMEI on the device by removing the battery, if applicable