The best way to look up a vehicle owner using a license plate number is through the Department of Motor Vehicles in the state where the vehicle is registered. If the owner is suspected of being involved in an accident or crime, a police report must usually be filed.
It is against federal law for a Department of Motor Vehicles to sell or provide current license plate owner identities to anyone but a licensed professional in law enforcement, an attorney, an insurance company or a government agency. There is always a fee for this information. It may be possible to look up the vehicle information (make, model and vehicle identification number) using online services in some states, but the information may be outdated. Some companies buy Department of Motor Vehicles information from prior years, which may or may not have changed or might not be in the companies' databases at all.