The first clue, for ME, is something as simple as the frequency of spelling errors and syntax errors. I know people will argue with this, but this is MY opinion. To me, if a person wants to come across clearly and intelligently, which I suspect most people want, they will take the time it takes to make sense to their audience. A person's whole argument can be lost amongst typos; and we all know how much the meaning of a sentence can be changed with a misplaced comma. Also, I think people will higher intelligence have a greater vocabulary and seem to be able to remember the proper spelling and usage of words most of the time.
Oh the irony!
Intelligence can be improved / enhanced / increased. I'm living proof.
But all the above matters not. Take Tom Cruise for instance. He can't even read but he's one of the greatest actors of all times, makes mad money and he seems to be very intelligent.