To find out who a car is registered to, contact the state DMV that matches the plates and request information on the owner. While some states offer this information freely, others do not. Such cases may require a request through the Freedom of Information Act.
Record all possible information about the car, including the license plate number, which state it's from, the registration sticker and the vehicle's make and model. If an accident or traffic violation occurred, states always provide the necessary information. Call and then visit the DMV that corresponds to the plates, in person or via its website, to discern whether or not that state can provide a name. U.S. Federal law considers such information as public record. If necessary, visit the Freedom of Information Act website for a list of agencies and directions on writing a request.