I guess this could be measured in many different ways.
Obviously there's general knowledge. People who know a lot about 'everything' have to be pretty smart.
But then there is the people who don't have much general knowledge, but know an awful lot about one subject. They are experts in there field.
One of my friends has a very poor general knowledge, yet financially he is doing the best out of all of us who grew up together. In fact, I would easily say he would be one of the top 15% earners, well above the national average. Basically, he has the gift of the gab! He can talk his way through anything. Though he denies it, I always argue that he has to be pretty 'smart' to be where he is.