Never ask a lawyer if it's OK to do something (it's always "no"). Ask them to explain the risks in doing so.

Never ask a programmer if something is possible (it's always "yes"). Ask them to explain what is involved doing something.

Never ask a doctor if something will make you better (it's always "maybe"). Ask them to explain the conditions under which a treatment/procedure will work and what could go wrong.

@davetron5000 I find with doctors (in an inpatient setting) the magic question is “what’s the next decision you’re going to make, and what information will help you make it?” Routes around their tendency to hedge when uncertain, because they almost always DO have a decision tree in mind (…the alternative is really scary) and feel comfortable discussing it.