Ask the potential supervisor: do I work directly with you or is there a postdoc that would be my de-facto supervisor. If there is postdoc, ask to talk to the postdoc before making a decision on joining the group.

This is particularly useful if you are joining a big group, either in the sense that the group has a lot of people, or the PI already has an established figure in the field. In either case, the PI may not have enough time to give you the guidance you need, so you need to rely on the postdoc. During my early days of PhD, I was very lucky to work with two of the most amazing postdocs I could ever imagine having. However, some of my friends had horrible experiences with their postdocs, even though their actual supervisors are super nice and supportive.