Dial *#06# on the phone
Check Settings > About phone > Status > IMEI information
Look on the SIM tray
Check the back of the phone or under the battery if removable
Find it on the original box or purchase receipt
Check the device label in the phone’s settings or packaging
For iPhone, go to Settings > General > About
For Android, go to Settings > About phone > IMEI
Check your carrier account or device account online
Contact the phone manufacturer or carrier if needed