Use About This Mac
Choose About This Mac from the Apple menu () in the upper-left corner of your screen. About This Mac shows an overview of your Mac, including the name and version of its operating system, its model name, and its serial number:
If you see an About This Mac window like the following, double-click the version number beneath “OS X” to reveal the serial number:
You can find the serial number printed on the underside of your device.