I think the most important is:
- Are you happy with your PhD in general?
It covers a lot of issues, but usually you don't want to do a PhD in a place where students are unhappy and frustrated. The good thing is that if students feel really bad, they rarely hide it.
The next things are related to:
- General contacts on the line student-advisor (How much contact and support can you expect, both for research and administrative stuff?).
- Funding (Is it a problem or a non-issue, e.g. for attending a conference?).
- Research (What the actual research looks like? What is the toughest part, biggest emphasis on, the most time-consuming part, etc?)
- How much time does it typically take to finish PhD? Does it happen often that someone drops out?
Other questions will depend of things you consider important. You may expect a lot of autonomy, or a lot of guidance. You may expect a very ambitious programme, or a PhD-life balance. You may like to teach, or you may like to keep it at minimum. You may be eager to travel a lot, or prefer to stay mostly in one place. You may be more into a particular branch of science or methodology, or into another...