How do you catch an eating disorder before it does further harm?

Communication is key. Knowing what obstacles and challenges your child is facing will help you determine if their behavior falls in the range of normal teenage angst, or if it may indicate something more serious.
Whether they like it or not, keep talking to your teen. If you think that they may be exhibiting any symptoms of an eating disorder, contact your doctor or a mental health professional to set up an evaluation.
You can also encourage them to call the National Eating Disorder Association Help Line at 1-800-931-2237.
Signs Your Teen Has an Eating Disorder — Sources
National Association of Anorexia Nervosa and Associated Disorders
National Eating Disorders Association
CDC
EatingDisorderHope.com