Check the original phone box or packaging
Look at the purchase receipt or invoice
Check the SIM card tray or back label if available
Log in to your Google account and check Find My Device
Log in to your Apple ID and check Find My iPhone device list
Check your carrier account or mobile service provider records
Check your online retailer order history
Check warranty or insurance documents
Ask the manufacturer support team with proof of purchase
Check any saved photos or screenshots that may show the IMEI
Check your old phone backup notes or device records
Contact local police if you need the IMEI for a lost phone report