Here are a few questions I would ask:
*Why did you go into medicine in the first place?
*Do you regret anything about your career path?
*What's your favorite part about practicing medicine? Least favorite?
*How much time do you have outside of work?
*If you had to do it all over again, would you still choose medicine?
These are some tough questions, so I wouldn't ask them all at once! And definitely shadow the PA too. Ask the same questions of his/her profession and lifestyle. A lot of people are finding that you can do many of the cool things an MD does by being a PA, with a high salary and better lifestyle (and two years of school compared to four). You need to make sure you know for sure that you want to be an MD and not a PA, and why! Shadowing a PA is a great opportunity to find this out.