Anorexia Nervosa is defined as a psychological disorder characterized by somatic delusions that you are too fat despite being emaciated. (wordnet.princeton.edu)

With anorexia, a person does such extreme things such as fasting (abstaining from food and/or drink) or heavly restricting their food intake.

Anorexia effects both males and females (but mostly females) in late adolescents to early adulthood.

Anorexia is not the same as dieting, although extreme dieting is a symptom of anorexia.

Anorexia may be accompanied by other psychological disorders such as bullemia (purging oneself of food by vomitting or extreme exercizing after consumption) or self mutilation (cutting).

So, here are some tips to watch out for in your child to know if (s)he may be suffering from anorexia nervosa.

1. Despite a skinny appearance, your child still insists on dieting. If your child is dieting and/or claiming to be fat even though (s)he clearly has a thin frame (perhaps even emaciated) (s)he may be suffering from anorexia nervosa. You may want to seek some professional counseling for the child to deal with any body image issues that may have arisen in the child.
2. Ritualistic behavior with regards to food. If you find that your child has ritualistic behavior with regards to food, for example: cuts up food into tiny pieces, will not eat anything of a certain color (eg. green or brown), does not eat foods when they are touching one another, then your child might have anorexia nervosa. Anorexics often times have hang ups about what they eat and how they eat it. 
3. Fasting. Anorexics often go several days without eating food. So, you may want to watch your child if you suspect he or she is developing anorexia to see if (s)he is eating anything. If you noticed that (s)he is skipping meals or is constantly "not hungry", you may want to intervene.
   

These are just a few things you may want to watch out for in your son or daughter if you suspect that they are suffering from anorexia nervosa. If you notice any of these warning signs. Don't wait! Get help! Talk to your health care provider about your concerns. You may be able to save your child's life.