When doing an interview, how much of the lab and the co-workers do one usually see? At the moment I would be lucky for any PhD offered, but I don't want to be stuck for years with people I don't like. What signs should I look for, telling me whether the department is good or bad? Any tips would be greatly appreciated.

At interview some departments ask a PhD student to show you around. Before you turn up for interview, ask if you will have such an opportunity. The atmosphere within any particular lab can make a big difference.

you could always ask to come back for a visit of the lab if you are offered the phd. at that point you already know they want you and are likely to say yes to you coming in for a visit and to talk to lab members before giving your decision. sometimes it's best to get lab members out of the lab (maybe go for cofffee or something) to get their real views...