The serial number is found on the back right side of the kayak.  See the image below.  It starts with QRQ  and the last two digits of the number denote the year the kayak was made.  For example if the last two digits are 19, then the kayak was manufactured in 2019.    It may be hard to see with the naked eye so running your fingers along the hull may be necessary. If it is not there it may be etched under the rear handle of your kayak.  For Riot inflatables the serial number is on the pod or fin of kayak or SUP.