An open and honest approach to gently confronting a child is important. Parents should approach an eating disorder like any other illness. It is a serious matter and they can communicate that to their children. They can also point out that there are professionals who will be gentle and supportive with them in proposed treatment. But many parents are faced with children who are openly combative and insist that nothing is wrong.
Parents can expect resistance and anger.
As you said, it is often part of the disorder. Taking a child to a physician can often be helpful. Because eating disorders also have a medical component, there are often telltale signs that will be picked up in a doctor's office. It is difficult for a child to refute medical evidence. In the case of a child's safety being jeopardized, a child may need to be escorted to the emergency room of a hospital where both a mental health and medical professional can assess the situation for safety.