I think if you have nothing to ask the medical school than you are not picky enough. I understand that, including myself, some applicants just "want to get into a medical school". Of course not everyone can be so picky. However, what is important to remember is that just because you got in, there could be things about the school/location/student body/curriculum that could make you miserable.
I think the best way to find questions is list things you DON'T want in a medical school, for example:
1. I'm a social person, I don't want a school w/o any social life.
2. I love the outdoors, it would make me miserable to not have any outdoors activities available to me in the area (fishing, camping, etc).
3. I learn best on my own, I wouldn't want to be in class for 6-8 hours a day.
4. I thrive in a supportive environment, does this school have a competitive atmosphere or a supportive one? Or both?
Anyways, I find that listing some things you don't want helps you to see what kind of questions you can ask. I also suggest to begin every question with a statement about yourself, for example regarding the question above:
" I like to know the weakness of certain programs so I can be prepared to overcome these weaknesses as well as come up with new ideas to fix the problem, what kind of weaknesses do you see in this medical school/cirrulcum/program"
or
" I like stay active as a way to manage stress, what are some EC I can participate in here at ________?"
This way you can give them a little blurb about yourself before asking the question.
Hope that helped