I think the best way is to ask them. You are not a child (assuming you really are 18 as your Yedda profile says), you are mature enough to know the facts even if you wish them to be different, and you deserve to get all the information.

Before you ask though, make sure you are ready to accept all answers, not just the ones you are looking for, because then your parents will lie to you to keep you happy and ignorant.