How can you tell when someone has an eating disorder?

ive recently noticed tht my best friend has lost alot of weight nad she looks absolutely terrible.shes 13 and has lost 3 stone since october 2010.lately my other best friend and i havent seen alot of her but wen she she is put with us she doesnt eat-nd we cud be out fer the whole day.shes also started smoking but i think its to stop food cravings.she admitted she is sick of herself and wants to be skiny but i jst want to know if other than this there are ways to tell if she does?