Obtaining information about other people's vehicle numbers is impossible unless by officials channels such as the state police. This is because such information isn't as readily available as you might think for security reasons.
If you want to know who the owner is because you are a little suspicious or just curious about the car that keeps driving by your home then the police may not help you.
I predict in ten years time this may change. But.......not today.
A private investigator can do this.
If the car's occupant committed a crime, the police can also check.
The comments listed here are of a general nature and should not be understood to create a formal attorney-client relationship -- that is, I am not, and I cannot give you legal advice here. I am licensed in Iowa and to become your attorney, I need to understand the particular facts and circumstances of your legal question.
Generally this is not publicly available information.
DISCLAIMER: I am an attorney licensed to practice law in the State of Iowa only and not in any other state. The response I have provided is based on the limited information stated in your question or hypothetical scenario. Quite often, a more complete understanding of the facts would lead to different advice or to a broader discussion of relevant or related considerations. The answer provided does not create an attorney-client relationship between us. You should not decide to act or refrain from acting based on my response, which is an attempt to provide general information on a legal topic as presented by you. The law is subject to frequent change and varies from state to state. Also, legal matters often involve deadlines that may significantly affect your legal rights. You should always consult with an attorney in your own state who is knowledgeable in the subject matter, and you should do so as soon as possible.