Method one: Check the product label on the external packaging of the product. The product label is on the side of the packaging, and it shows basic product information including the model number, as shown in the following figure.

Method two: Check the nameplate inside the back of the phone. This is only possible if the rear cover is removable. If the battery is removable the nameplate will be behind the battery.
The following figure is for your reference.

Method three: Check the model number in system settings. Go to Settings > About phone > Model number .