What should I ask my potential Ph.D advisor in advance, before actually signing a contract?
I would add:


What the average time-to-PhD is, and how many percent roughly drop out (although this is a question that is much better asked to other PhD students, as you may easily hear somewhat "tuned" numbers here)
Paper authorship policy (via mhwombat's comment) - however, note that theory and practice may diverge quite strongly in this topic. This is again something better asked the colleagues.
Grant / project duties - if you are e.g., paid by an FP7 or H2020 project, your project work may cost you much more time than teaching duties
Administrative duties
How free you will be in choosing whom to collaborate with, and what topics to collaborate on
Whether you will be required to work on other projects on the side (e.g., consulting work, paid research, etc.)
Whether you will be expected to help with writing grant proposals (time sink number one if yes)
Whether there is a culture for working from home
What office / lab space you will get, how many other students you share it with
Whether it will be possible for you to do an internship, e.g., over the summer.


There may be more. I will extend the list once I remember more important things.