[gbaygon]

The definition of stupidity is "being lack of intelligence", and the definition of intelligence is "the ability to acquire and apply knowledge and skills".

So maybe a stupid person can only be detected by observing said person during a period of time and measuring the ability to learn from mistakes and update behavior. If the variation on behavior is zero we can say we spotted a stupid person. But, if the variation is positive we are dealing with somebody "learning" and maybe we can "help".