Tutor people in things. (You have to be smart to teach other people things.)

Definitely this. Tutoring is a very strong signal of intelligence, but is really a matter of learned technique. I was able to tutor effectively in Statistics before I had taken any classes or fully understood the material by using tutoring techniques I had learned by teaching other subjects (notably Physics). The most common question I found myself asking was "what rule do we apply in situations like this," a question you do not actually need to know the subject material to ask.