Frankly, all you need to do is make a list of the model numbers (given in a link in this thread) and have that with you when you check. Measuring with a micrometer would be a waste of time, IMO, and could possible lead to damage of the device. It would also take longer than just looking at the model number. Punching in a serial number is also a rather slow way to do this. Also, the model numbers also back of the box. If I were spending money, I'd look on the back of the device itself. Also, I would not buy it without verification that it comes on.
The serial and model numbers are on the back of every iPad, as well as viewable in Settings.