I agree with the post above me, i.e. if you take "past" to refer to not just love history, but his upbringing, the friends he hang out with, how he treats his family etc etc, then I understand that we should learn more about him from them.
However, we don't question him to get all this info, do we? We observe and come to the conclusion (be it good or bad) by ourselves, or perhaps find out from mutual friends, etc.
Even if we do question him about his past, will a person with "dodgy past dealings" tell you honestly? I am more inclined to think that a truly dodgy guy will just lie in an attempt to pacify us, while those more honest guy who really tell the truth about his past will affect his current relationship. Lose-lose situation.
That is why we say open both eyes during dating. Action speaks louder than words. Observe how he treat his family members, interact with his friends, how he treat you (esp the little things), and there you have it.
Cheers!