One solid piece of evidence is whether faculty have published with each other. Relatedly, you can see if they are co-advising any students. This can apply to teaching-focused places, too, as there are thesis students.
On the interview you can look at how they interact with each other and whether they bring up personal things with each other, (e.g. "are we on for that movie Friday night?") Though I wasn't looking for it in particular, I did notice certain interchanges during my job talk for the place where I am now (which is collegial). The room was bigger than needed with lots of empty seats, but faculty sat down next to each other. One professor kept whispering to the other prof next to him and pointing to things in my talk (that sounds bad but it was stuff he was impressed with, as the questions revealed.) It was clear they were friends. I also noticed another clump of professors talking with each other before the talk started, seemingly about "life" stuff. I could tell they were close as well.
I don't think the absence of these things is a bad sign but the presence is probably a good sign. The only thing that could still be going on in the above scenarios is if there are separate cliques in the department.
If you're trying to sniff out a "bad" department I think the best thing you can hope for is to find the one honest colleague. It might even be the out-of-department search committee member if there is one.
Once they offer you the job you can grill people more, too. You can even ask to speak on the phone with the person who was conspicuously not there on interview day.