The serial number on a tablet is frequently found on the body of the device, printed on the packaging, or within the settings of the tablet. The location of the serial number varies by device and model.
All Apple iPads have a serial number engraved on the bottom of the back of the tablet. Additionally, Apple prints the serial number on the packaging of each device. Another way to access this information is within the "about" section of the general settings.
Microsoft's Surface tablet displays the serial number on the back of the device as well as on the original packaging label.
Other tablets such as the Kindle Fire do not show the serial number on the device itself but do list it within the "device info" section of the tablet settings.