Pull back the rubber flap covering the ports on the back of your speaker. The serial number is printed on the inside of the flap. It begins with RAVE, followed by more letters and numbers. Learn about the Beats Pill XL speaker recall program.
Photos below show the models and where they are marked. All models made by Apple (post 2014 manufacturing) have a model # located on the inside of the right hinge. Model made before that have lot numbers which are located on the bottom of the left has side on the swivel or inside panel. Having a serial number does not necessarily mean your pair is authentic; here's an article helping ID fake Beats.
Beats Pro Detox Serial Number Check