This may be country specific, but there are a couple of points I would make.
First, in the (UK) departments I know, there must be more than one faculty member involved in the supervisory team, and a completely inexperienced faculty member would not be allowed to lead (officially). It would be worth checking beforehand if there's a plan to have more faculty involved in the supervision, in what roles, and at what level. If possible, suggest faculty yourself (this could be difficult to do tactfully, as it may suggest to the PI that you don't trust their abilities and expertise: I would suggest something like "With our project, it might be interesting to consider doing X, but I know nothing about it. Could we involve Y in the supervision from the start so I can learn?").
Second, there are a number of directions that a lead supervisor should push a grad student on (such as day-to-day research, big picture research direction and future careers, profile raising activities like talks or posters, communication activities like papers, administrative issues and hoop-jumping like performance reviews or qualifying exams). The advantage of experienced supervisors (IMO) is that they are better at knowing what to push on and when. When less experienced, supervisors can put too much weight on everything at once, leading to too much pressure on the student.
One step to checking this is to have more experienced faculty involved with the supervision, as above. Another, particularly for the administrative hoop-jumping steps, is to have a strong network amongst the other grad students in the department or faculty. You mention that the PI has no other students: I assume the lab, or department more broadly, does. If so, make it a priority to connect with them, to find out timings and expectations, and build a support network. If there are no other students in the department I would say you're taking a very big risk joining the lab.