I am finishing up a Ph.D. in Computer Science, and have been involved in student panels at PhD student recruiting many times. In a lot of cases, the incoming recruits are straight out of undergrad and don't even know the "right" questions to ask about a PhD program/school when evaluating where they want to go. To help people who are evaluating a CS PhD program I have developed a list of the questions that you should be asking. Many of the questions are not specific to CS, and can be useful to ask at any PhD program recruiting event.

Questions that somebody in the administration should know the answers to:

1. What is the average time to graduation?
2. What is the ratio of PhD students to Professors?
3. How much are the mandatory fees that I will have to pay out of my stipend?
4. Does that include health insurance?
5. How is summer funding handled?
6. How long is funding guaranteed?
7. What are the steps that lead to a degree, and how do you complete them? Classwork? Comprehensive or Qualifier exam? Proposal?

Questions that you should ask current grad students (preferably without faculty around):

1. Who was the last faculty to be denied tenure? Why were they denied? What happened to their students? 
2. How well do the faculty support their students? How about the administration?
3. Do the faculty provide support when looking for summer internships? What internships have current students had, and with what companies or labs?
4. Does the school or faculty pay for students to attend conferences? What type of paperwork do you have to fill out to get a conference trip reimbursed?
5. How is the computer support/networking/IT department? Are students provided computer when they arrive? Do the printers always work? Does the campus/department email work?
6. How much does housing cost? Where do current students live?

Questions you should ask the current grad students of a professor you are considering:

1. How frequently do you get to meet with your adviser? Are the meeting scheduled in advance? How frequently does the adviser cancel them? If canceled, do they re-schedule?
2. How many students has this professor successfully graduated? How many years did they take to graduate? If the professor is relatively new, how far along in the program are their current students?
3. What is their work schedule like? Do they come in early? Stay late?
4. What is their working style? Do they micro-manage? Do they set large goals and check in once a month?
5. (If they are a new faculty member) Do you think they will be granted tenure?
6. (If you are a woman, or care about equality.) Has this professor graduated any female grad students?