This article applies to all YubiKey and Security Key devices.
To identify the version of YubiKey or Security Key you have, use YubiKey Manager, which will show you the model, firmware version, and serial number of your device.
The serial number, which is printed both literally and in 2D barcode form on every YubiKey is the definitive way to uniquely distinguish one YubiKey from another. Serial numbers are unique across all models of YubiKeys, with the exception of Security Keys, which do not have serial numbers. A YubiKey serial number cannot be changed or overwritten.
For physical identification, please refer to https://www.yubico.com/products/identifying-your-yubikey/.