We can usually recognize intelligence, but it is hard to define what it is.
e.g., the PET Scan. Try to look at the brain and figure out how it works. Presumably success would reveal the source of intelligence (and its "twin", consciousness).
Enumerate the properties of intelligence as Hofstadter does.
Devise tests for intelligence, e.g., the IQ test. This test does not measure absolute intelligence but just relative intelligence.
IQ tests imply that there is an abstract universal "thing", intelligence. IQ tests purport to quantify amounts of this "thing". But maybe there is no such thing. Maybe there are different intelligences (or talents).
Contrast the mathematician John von Neumann, who probably had one of the highest IQ's in history, with the French Impressionist painter Claude Monet, one of the greatest painters in history, who was definitely not a rocket scientist. Two extremely different intelligences.