First, Intelligence (as probably this article says) is an ever evolving concept and its multidimensional. Accurately measuring it seems probably impossible, the least really hard. The way we attempt to measure it, is via standardized tests - but those don't really prove much because instead of really measuring what they should measure, they usually end up measuring "test-taking" abilities.
Just think about Einstein and Leonardo da Vinci for that matter. Einsten did terrible in school and would have probably done terrible on an IQ tests. Leonardo da Vinci (arguably the greatest genius of all history) was rather slow in problem solving, but increadibly effective. He would have probably done terrible on a timed standardized test.