You will find the serial number engraved on the bottom bracket between the pedal cranks. You may have to lift the bike to get a better visual. We advise placing a towel down to avoid accidental paint scratches. Please see the attached photos for reference:

For models RX, S2, ZX and Z Miami:

For the SG1, ZG, and OG models, you can find the serial number on the top side of the bottom bracket, as pictured below:

OR

On the underside of the bottom bracket: