A part from the obvious discussion on salary, TA or other forms of support, here is a list of relevant questions you should be asking your future thesis advisor:

• Will we meet on a regular basis (group meeting, individual meeting)?
• Is there specific graduation conditions i.e. must have published at least 3 manuscripts in the top journal of the field ?
• Publication policies: will I be first authors?
• In your group, under what condition graduate students get to go to meetings/conferences?
• Will I get the chance to supervise an undergraduate student?
• What will be the infrastructure (field specific) available for my project through your group / Department / University?
• Is there any possibilities during my PhD to able to spent a few months in one your collaborator’s lab (if advisor has ongoing, established collaboration)?

Note that before getting to the point where you will be asking the above questions, you should also do your homework and research your thesis advisor activities, realizations and past grad students:

• Does he/she publish on a regular basis in the best journal in the field?
• Learn about past students: how long to graduate on average and what are these students doing now?
• Who are the current students and what are they working on (how will you fit in this group)?
• Does this group has a website and is it up to date?