In our group, potential new starters are more than encouraged to talk to current postgrads - for example we normally take them out to lunch when they come to interview/present, without the academics in the group present. This works both ways - a group that works as a team will want members that fit into the team. If you are asked to present to the group at interview, that's inherently a good see (IMO), but the tone of a question session after that is a good gauge - not the supervisor's questions, but how the rest of the group ask questions in front of him/her.
You can get a feel for the dynamic of the group without asking directly - find out about whether they do stuff together out of work, for example.
Checking the order of authors as suggested by dmvianna is a good idea, but you need to do that in the context of the field - e.g. in my area of applied physics, postgrads generally do experiment+analysis+writing and are first author. My perception of particle physics is rather different.
Your worry about being isolated is certainly valid, I feel very lucky in the group and department I'm in, when talking to people elsewhere in the university they are often quite lonely in their work.