The majority of serial numbers are located under the bottom bracket where the two pedal cranks meet. Simply turn your bike upside down and record the number. If there is no serial number there then check places like the headset at the front of the bike or the rear stays. The diagram indicates the five most common serial number locations. If you just can’t find anything then you can either engrave your driver’s license number or describe the bike the best you can when registering and maybe even take a picture with it to prove ownership.
The best chance at recovering a stolen bicycle is having a registered serial number. These numbers are used by police across the nation. You need the manufacturer's serial number to register your bicycle.
Where to find your serial number:
Locations of frame and components with serial and frame numbers: