You can't. This information is recorded by the department of motor vehicals and widely used by the the police. Unfortunately, neither of them will release the information unless you know them personally as that information is protected.
Call the police and explain why you need to know. If you are looking for a legitimate reason they can look up the number in their database. I do not believe that database is open to the public.
take it to the dmv. or get in touch with your secretary of state.
I think the laws vary depending on where you are, but in many places you have to be in a position of certain authority in order...
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