As Andy says, qualified professionals. But I think you'll find that there isn't always consensus among them.
And yes, it varies from country to country.
My facts are rather out of date, but when I did my degree (psychology) we looked at the fact that the US system (known as DSM IV - Diagnostic and Statistical Manual of Mental Disorders) resulted in more Americans being diagnosed with schizophrenia than they were in Britain, as the criteria for diagnosis were much wider.
Definitions of acceptable behaviour and what constitutes mental illness also vary historically. Remember that less than 100 years ago you could, as a woman, be diagnosed as mentally ill (and locked up) for liking sex. People whom we would now consider to be mentally ill have in the past have been regarded as wise and even visionery (e.g. Hildegard of Bingen) - and note the number of revisions of the DSM (now writing number five!) and the fact that homosexuality was at one time listed as a mental illness!
The definition of what is mentally ill is a very moveable feast...
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