They use isinstance, except for a couple that also check co_flags, and the ones that check if the object is a descriptor.  I haven't thought this through fully, but I think this means that in general the descriptor protocol has been invoked or not by the caller of inspect before inspect checks the object.  There is no 'callable' type in python, so the closest analog in the inspect module to 'callable' are the functions that look for __get__ and __set__ methods on descriptors.  If one of *those* is a descriptor, my head will start hurting :).