Or if I were to try the existence of God, I predict half the population would say it's 100% certain God exists, and the other half would say the opposite.

So this might be the result of huge selection effects, but most "average or somewhat-above-agerage" people I've met, e.g. people at my high school, were agnostic, and weren't very mind-killed on the subject; in fact, they were mostly disinterested, and wary of both theistic and atheistic evangelism. I think the God question would work just fine for such people.

ETA: Ah, Vaniver already said similarly.