Elitedesk PC (May be mounted on the back of the monitor display - if inaccessible use other method below.)
You will find it in small white letters on bottom of black box.
Another method while being logged into the Windows device:
Right click on the Windows icon on the bottom left corner of your screen.
A list will appear, click on System
In the middle of the screen look for Device specifications
Device name is your serial number.
You will find 2 identical stickers, one on the front and one on the rear.