Determining who holds the copyright is obvious when the work is recent and has a copyright notice. But sometimes it becomes detective work and may take time, and scholars need to be prepared for this.
Copyright notices (©1963 Jane Doe) traditionally served to identify initial copyright holders in publications but they are no longer a required on published works in the United States. Copyright notices were never required on unpublished works. Also, over time, copyrights may have been transferred in part or in whole to other people or entities, or inherited by heirs. The older a work is, the harder it may be to identify the current copyright holder or copyright holders.
There is no comprehensive national or international registry for copyright ownership, transfers, or inheritance. The copyright registration records of the U.S. Copyright Office may hold information. Other helpful sources are the WATCH database, a database of copyright contacts for writers, artists, and prominent people in other creative fields, and the Firms Out of Business database. The websites of ASCAP, BMI, and SESAC are a source of information for music compositions in the respective repertories. In sound recordings there are copyrights in both the recording and the underlying musical compositions. In films typically the production company holds the copyright through contracts signed with contributors who were eligible for initial copyright, such the director, the scriptwriters, and the musical composer, and with performers. Some investigation may be necessary.
It is not always obvious who holds copyright in a work. Scholars need to allow time for investigating works if they need to obtain permissions. Keep in mind that it is not necessary to obtain permissions if works are being used under a copyright exception or if your publisher agrees that permissions are not necessary.