Serial numbers are usually found on the underside of laptops.
The problem here is that most people do not know there is a serial number under their laptop. The only way to find out without tearing apart your laptop and finding it physically yourself would be to pull up your device details and go explore at least one level. The downside to this solution, though, is that you have no guarantee you will get the right model or even accessory information because there could be all sorts of other information in those layers. So if you're having trouble locating a serial number for any reason just google "laptop manufacturer r and letter and the words serial number" and do some digging around different manufacturers pages until something pops up under your search parameters."...