I do think that is sometimes difficult to accurately diagnose a child for any kind of developmental disability. In some children, it might be more straightforward. Certain children might exhibit more obvious or straightforward signs or "defects" In other children, it might just be a child who is a little bit different than their peers and not necessarily disabled.
I believe diagnosing a child with a metal retardation can be comparatively difficult. While medical studies have shown that children tend to meet certain criteria at certain ages it is in no way indicative of all children's experiences. Since these diagnosis are usually gathered by comparing the child to others in his peer group, they can sometimes lead to false conclusions.
Yes, for a child to be diagnosed with some type of mental illness, a long screening and lots of research is done by a trained professional. They know when to diagnose some one with a mental illness, and know the best ways to work towards the child having a normal life.
I think diagnosing a child with a mental problem of any type would be difficult at first and hard to recognize. All children act a little goofy, a little hyper, and probably do not know a whole lot. That makes it hard to recognize signs of a struggling child early on.
IQ is fluid in many senses of things and IQ tests are hard to give to kids in certain areas and backgrounds, but actual retardation is fairly straight forward. People can base a diagnosis on a set of physical characteristics and or competency tests and experiments in the real world.