LibraryLaw (an excellent blog, by the way) reports to us that the Copyright Renewal Database at Stanford is available. It covers those key dates, 1923-1963 (read the About the Collection section of the web site).
And if you are a neophyte just embarking on a copyright quest, don’t forget to start with the official U.S. Copyright Office web page. Everyone has an opinion on copyright law. Read the best, read the law, do your research, and when in doubt, consult an attorney.