What is your approach to reference checks?

First, although I'm not in HR, I like to do my own reference calls rather than have HR do it separately. I may have HR on the call, but I want to get a feel for how the referenced person is responding. I call 2-3 of the provided references and use my network (usually via LinkedIn) to find someone I know who may have worked with the person in the past or knows someone who did to do a back-channel reference check. For the calls to the candidate-provided checks, I try to keep it to a casual conversation and ask simple questions that let the referenced person speak freely. If they're short or terse, that's a warning sign to me (but may just be a personality trait, so it needs validation with other calls). I find that a natural conversation rather than asking a lot of questions reveals more about my candidate. By the way, in my opinion the same holds true for interviewing in general.