Whenever it becomes necessary for the purposes of this article to determine whether a child who participated in a sexual performance was under an age specified in this article, the court or jury may make such determination by any of the following: personal inspection of the child; inspection of a photograph or motion picture which constituted the sexual performance; oral testimony by a witness to the sexual performance as to the age of the child based upon the child's appearance; expert medical testimony based upon the appearance of the child in the sexual performance; and any other method authorized by any applicable provision of law or by the rules of evidence at common law.