The truly intelligent one will be the one who knows eveything.
Remember Sage Uddalaka asking his son Shwetaketu "Do you know that which by knowing it one knows everything"
Is there anyone in this world who is "truly intelligent" ??
Except GOD perhaps, the all-knowing one, i can't think of mortals who are truly intelligent by that definition.
But i do accept the theory of multiple intelligence. Being knowledgeable about one thing and not-knowing other things does not make a man less or more intelligent. Each has his own set of abilties.
Intelligence cannot be judged. Nor can genetics predict it for sure (atleast not as yet). Everyone who carries the Y-chromosome of Albert Einstein , like his paternal cousins, or his X-Chr like his maternal cousins, did not show the same intelligence level as einstein.
Seers of the Rig Ved also wondered whether God exists or not; showing that they were not "all-knowing" ones either. Some seers confirmed God exists, some confirmed God does not exist, some say God is the self, etc. Each of those "confirmations" are based on how each felt.
Intelligence, is truly a very relative term. No one, no matter how spiritually advanced either, is infallible. Being all-knowing is humanly impossible, i suppose...