I have to be honest, I'm a little suspect when one of the first questions a founder asks me is about valuation. So many things about startups are difficult and so few startups raise *at all* versus the number who try, that this seems like not the most important question.
That being said, you want to feel like you got a good deal--and your lead investor should be able to walk you through how they got to a particular valuation and why they thought it was appropriate. They should be able to provide examples of other deals, and talk openly and transparently about their thinking.
Look, at the end of the day, I'm incentivized to buy up as much of your company as possible, and you're trying to sell as little of it as possible. On valuation, we're not aligned, but we can meet somewhere in the middle, both feel a little regretful about where we wound up, and that's probably the right price. :)