In my opinion, intelligence is multi-dimensional. There are many ways to display intelligence, whether it be in writing, art, mathematics, or some other method. As such, I'm sure different people use different markers to determine the intelligence of others.
Personally, I judge by a person's ability to communicate (I would not recommend this method. It's very flawed and frequently makes for confusion in forums such as this, where I instinctually judge intelligence on grammar and sentence structure). I'm working towards judging by quality of ideas, instead of how they are presented.
What about you? How do you judge a person's intelligence? Does it even matter to you at all? SHOULD it matter?