The notion of a genius is relative. In order for someone to be labelled as "genius", they have to possess incredible talent in something that is widely recognized as being significant. Say, in the prehistoric age, someone who could hunt meat better than anybody else would be considered to be a genius. However, the expected skillset has changed. Because the societal expectations have become academic, someone with an amazing ability to link concepts to create something new would be considered to be a genius. Someone who is amazing at mental math could be considered to be a genius. Someone who could flap their ears or could whistle well, however, would not be considered a genius, because even though these skills can entertain people for a while, they are generally useless for the society. And as Danikov said, we use IQ tests to figure out where along the intelligence spectrum a person falls, because we do not have a more adequate measurement of intelligence. And there would never be a fully accurate measurement of intelligence, since we can only test specific skills and such an intelligence test would require an infinite amount of skills to be measured and added up.