How can join authors establish the value of each contribution to the work?

The Copyright Act of 1976 defines "joint work" as creative work prepared by two or more authors working under the intention that their contributions will be merged into a inseparable or interdependent parts of a unitary whole. This Copyright Act does not specify guidelines on how to value the different contributions in joint works. However, common-law principles, court decisions, and the writing of legal commentators agree that when there is no agreement between the authors, a court will presume that each author had an undivided interest in relation to the total number of authors (i.e. the value is divided equally among the authors).

It is often the case that not all the authors of a copyrighted work contribute equally to the work. When multiple authors write of a textbook or a band that writes a song together it is often the case that not all the authors contributed equally. In these situations it is important for the authors to sign a contract where they specify the rights, obligations, percentage of copyright ownership, and revenues attributable to each author. This type of contract is known as a "collaboration agreement." Collaboration agreements are very common in the creation of works such as books, software programs, screenplays, and songs.

References:
[1] Stim, R. "Intellectual Property. Patents, Trademarks, and Copyrights" West Legal Studies.
[2] Black's Law Dictionary 5th ed., (West Publishing, 1979).
[3]
Copyright Act of 1976