On a Plate fassened to the driver's side door post. Open the driver's door, look on the frame and post junction, the plate will be 6 to 8 inches up form floor.
Copy the VIN number off of the dash, door jamb, or registration. Go to here: http://www.forabodiesonly.com/mopar/decode_vin.php
Depending on how old a car is, the information you find from a VIN check may or may not be updated. If a car is less than five years old, it is typically kept in check with the dealership it was purchased from, which would ensure that its VIN check is reliable.
By searching with the vin number
On Old Cars.....
If the car is still in state and the owner has not opted out the state can run the VIN and tell the current owner. In Kansas it's the Dept. of Revenue that does it, it may be another branch in other states.
If you have the VIN you can go to your local DMV and have a search done. I must warn you that the search can only be within the state that you are in now.
Old tag number, I doubt it, vin number, maybe.
The vin# should also be on the block near the top just under the valve cover gaskets. Depending how old the car is it also may be near the oil pan.
If you still have the VIN # you could possibly contact the DMV to find the location, or you could ask the person's old friends to see if he/she left a contact address.
There are two places to look on the car. the drivers door post has it on a sticker. That does come off. There's a metal plate on the dash on the drivers side. You have to read it from the outside. If the cars not avaible, go to where you pay your taxes and ask them for a list where you've paid taxes on it. It might be there.
You can't.
It is not possible to register a car in California with no VIN or title. The owner of the car must produce a valid title for registration. If the car is too old and never had a title or VIN, the title can be applied for at the time it is registered. If the title is lost, a new duplicate title can be issued to the person named on the title.