Questions to a prospective PhD student
In order to know if the candidate is a good cultural fit for the group I usually talk more about the working atmosphere of the group. I usually focus in explaining how things work in the group; what I do (project, year of the PHD program), what I studied, what other team mates ask me to do, what the boss encourages.


Sounds right on the money to me. Sometimes it's hard for people to be self aware enough to answer these sorts of questions, but you should ask about exactly those topics: 


what sort of working environment do they operate well in (do they work best in a quiet room or a busy room? working individually or closely with another person/group?), 
level of supervision (how often do they expect to meet with a PI? do they want to work mainly independently or be given tasks to do?), 
how do they like to receive feedback, social preferences (do they like to go out after work with colleagues or head straight home?), etc. 


You can then help them compare their desires with what your actual experiences are in the lab.

Ask them what their concerns are coming into a PhD program, what makes them anxious about a new working environment, etc.

Also, it shouldn't be just about asking questions but also providing information. The prospective students should be interviewing you as much as you are interviewing them. Let them know what you like about working in that lab, what you don't like, what you have been able to change or what can't be changed.

Importantly, none of these things are really about the quality of a candidate but rather about their fit. This stack is full of people will all sorts of problems mid to late in their PhD that boil down to them joining a lab that was a bad fit. It's important for both the lab and the student to make a good fit.