VIN (vehicle identification number) can be found on the vehicle title, registration or other documents, as well as on the vehicle itself. VIN consists of 17 characters and ends with 6 digits.
e. g. JTHBJ46G2923XXXXX
On the vehicle itself
In the documents
Contact the seller
In case you are unable to get a Vehicle Identification Number from the documents or the car itself, contact the seller and ask him to provide you the VIN number of the vehicle.