Before deciding to do one's PhD under a certain professor, what questions should one ask of his/her former and current students?
Many students (including me) are not sure what exactly to ask. May be a community wiki here might be a general source for such students.
Please recommend phrasing of the question(s) also.
This is a related question, How to evaluate potential advisers on grounds other than their research/publications? but it is much broader. I'm looking for specific questions to be asked to the students. Also manner in which to ask :)
P.S. I have accepted an answer, but feel free to add more answers if you think they differ than the ones already given.