In my personal opinion, take him shopping, and without giving too much away, take him to the jewelry section of the store. Then slowly browse through the jewelry until you reach the wedding section.
Then ask him: If you were to propose, which ring would you buy me? Some men answer, others just ignore the question.. If he picks a ring, ask to try it on, and watch his reaction!
My boyfriend actually did this to me about 8 months after we started going out, it came as a shock, but surely enough, on our one year together, he proposed with a ring similar to the one he had me try on at the store!