Joining a group for PhD, how do I judge the atmosphere before I sign?
First off, if the professor is reluctant to have you talk to members of the group, this is a huge red flag. 
When you visit, ask if you can sit in on lab meetings or seminars (some group setting) and observe the dynamic. Does everyone seem engaged and happy to be there? Is the mood collaborative and friendly, or authoritarian? Basically, put yourself there and ask yourself if you'd be comfortable.
Ask the professor about their advising style and philosophy. Some like to be very involved, some want their students to be independent. Make sure their answer works for you.
Most important one, in my opinion: contact a few of the other students independently, tell them you are coming to visit, and offer to take them out for a coffee or beer. This is an opportunity to ask them, in a relaxed non-work setting, about what they like and don't like about working in the lab. Do not tell the professor what the students are thinking, in any case.