I applaud this article!
How Monitoring Your Child's Pediatric Growth Could Help Prevent An Eating Disorder by Dr. Lauren Muhlheim.
This is thorny: parents need to know that they can't tell by looking at someone whether they have an eating disorder. But you DO need to know your child's normal growth pattern.
It isn't that weight is the only symptom, but parents you do need to know your child's growth trajectory. You can not tell by looking if your child is failing to gain height or weight due to subtle restriction. That restriction could be from a number of reasons, as the article states, but it is not normal and may be an early sign of mental illness.
The best time to intervene is as soon as possible. For those predisposed to these deadly mental illnesses you may be able to intervene early enough to stop it in its tracks.
Believe me, you do not want to wait until you can see it. And this is not just anorexia nervosa, this is all eating disorders: restriction is the beginning of bulimic and binge eating patterns as well.
Monitoring growth isn't everything, but it is REALLY important!