For example, the question I was asked when I looked into posting a comment was:
What color is an orange?
The key of course is to make sure the questions are random enough that spam bots cannot guess them easily and their logic would need to be greatly improved in order to have a chance at success. Also, which is probably far more important, the questions cannot be too complex. I know if I have to think about an answer for more then a second or two, I maybe less likely to post a comment on a site. For those who don’t comment on blogs often, they might even be less willing to figure out an answer.