never seen a derm myself, although I'm sure my questions would be something like these:
Is it falling out? is it male pattern baldness? is it aggressive?
I would think it would be a good idea to know your family baldness history and patterns as they may ask , also the speed of anything you lost so far.
basically just explain your situation to him/her.
after the answers or diagnosis then you can ask about recommended treatments for what ever condition it is (if any).
you can find most on here anyway and possibly if they mention propecia I think there's a test they can do to see how you will respond, so you could mention that.