604(b) evaluators are usually child psychologists or psychiatrists, often with experience in child custody issues. They are a third party; they don't represent one side or the other. The evaluator is meant to offer a neutral opinion.
Interviews and document review
The evaluator will spend a significant amount of time with the child before making a decision. The evaluation process also will include interviews with both parents, and may include interviews with other adults in the child's life. The evaluator also may consider documents related to the custody issues.
The evaluator's job is to determine what is in the best interests of the child. After considering the needs of the child, as well as any other information gathered in the interview process or from various documents, the evaluator give their opinion. The recommendation is usually put in writing.
The judge will receive the evaluator's opinion. However, the evaluator's recommendation is not set in stone. It is just one of many factors considered by the judge in making a custody decision. Also, an evaluator can be called as a witness at trial and questioned about their recommendation.