Typically a custody evaluation includes more than 20 hours of interviews and tests. Parents and adults are always interviewed in depth, typically at least twice each. Everything that might effect your children is fair game however. Additionally the evaluator will meet with your children, and the older and more mature the children are, the more time the interviewer will devote. Observations of you and your children interacting are important.
Many evaluators develop specific observations built around tasks, while others just observe.
Psychological testing will often be done of the parents if there are any serious questions raised about the mental health of one of the spouses.