If the only clue you have to someone's identity is their license plate information, you'll have to do a bit of sleuthing to find him. In many cases, license plate information is protected in secured databases and is only available to licensed investigative professionals. Don't despair, though. There are some back-door methods to finding a person based on his license plate number, including internet searches and accident reports.
Write down the details of the license plate and the car as soon as you can. If there is anything uncommon about the vehicle, search your local area and the area where you saw it on a regular basis. Most people have daily routines that keep them returning to the same places, and you're likely to find the same car in a nearby parking lot if you remain vigilant.
Find a local private investigator and hire him. Private investigators often have access to databases of license plate numbers or have made contacts with local law enforcement who may be willing to find the plate number for them. Unlike police officers, who may not give you the information you're seeking, private investigators are working to get you answers.
Sign up for a service that offers license plate recognition and input the information you have. Companies such as #1 License Plate Search, Reverse License Plate Search and Abika offer clients the ability to search their databases to find the owner of a car based on her license plate number.
Go in person to your state's license registry or Department of Motor Vehicles if you were involved in an accident with the car in question. They may be able to provide you with the person's name. Though you will not be able to get the person's contact information, having his name will make finding him much easier.
Post an ad on a local bulletin board, such as Craigslist, offering a small reward for anyone who can identify the owner of a car with the license plate number.
Do not break the law in an attempt to find out who owns the car with the license plate.