I do agree that by the time you get to a very low weight you are completely under the control of the illness and so it is much harder to treat, but until that point it is hard to diagnose who is truly anorexic and who is just losing weight or those girls that act like they're not eating for attention. However anorexia isn't exclusively about being thin. Starvation is control, control is tough but bones are beautiful when thin isn't enough.
Also you can have cancer for a long time before it's diagnosed, so I suppose the same can be said about anorexia, there just aren't definitive biological tests. You can't scan someone's brain and tell if they're hearing voices telling them that if they eat they're a failure and don't deserve to be happy.
To be honest I should probably leave this thread, it took me a long time to accept that being able to see my bones through my clothes wasn't a good thing and I needed to gain weight and be okay with being a 'healthy' weight, all this talk of being fat is getting to me a bit.