I normally don't care about a person's intelligence unless I'm expecting something from them or need to explain something to them. I do so by asking questions. If they've irritated me, I'll make them extremely simple questions. Ones that are most certainly going to annoy the other person.
If they haven't annoyed me, I'll explain something as simply as I can without being offensive.
Sometimes I'll take a stance entirely different than one I actually believe in just to get a person to see a point. Sometimes it works, other times they get all pissy and offended. I guess I can be rather intense without meaning to be. Sometimes they just think I'm some rambling idiot and ignore my points altogether.
It's like throwing a fast ball and the other person is on the plate, batting. Some people will swing at the ball and miss. Some will not even SEE the ball and just stand there with stupid expressions on their face. A few people will see the ball, see your point and hit a homer.
When that third thing happens, it's truly a wonderful feeling on my side.
"YES! THEY GET IT!"
If I find they know more than I am, I start asking questions.
I would never pretend to be stupid to incite an argument. I only ever feign ignorance when I've come to a conclusion that I'm not entirely certain about and need to fish about for more information. Usually, that only happens when dealing with people and their emotions.