Locate your Serial Number, Model Identifier, and Part Number
MacBook, MacBook Pro, MacBook Air, iMac:
Go to Apple menu > About This Mac. You’ll find your computer's model name and serial number.
Go to About This Mac > System Report > System Information window Hardware Overview. You’ll find the model identifier.
Serial number is also located on the underside of the computer, barcode on the original packaging (as well as the Apple part number, ex: MLH12xx/A), and may be on your receipt.
Sources: https://support.apple.com/en-us/HT201608; https://support.apple.com/en-us/HT201300; https://support.apple.com/en-us/HT201862; https://support.apple.com/en-us/HT201634
iPad, iPad Air, iPad Mini, iPhone:
Go to Settings > General > About. You’ll find the Model number, Version, Serial Number, Capacity, and other specifications about your device.
There is also a model number engraved on the back of your device, but this will not give you storage capacity information.
Source: https://support.apple.com/en-us/HT201296
We also suggest using an online database such as Everymac.com where you can input any device identifier to find more information on your device.