  Different criteria (Shannon's entropy, Bayes' average cost, Durr's normalized
rms spread) have been introduced to measure the "which-way" information present
in interference experiments where, due to non-orthogonality of the detector
states, the path determination is incomplete. For each of these criteria, we
determine the optimal measurement to be carried on the detectors, in order to
read out the maximum which-way information. We show that, while in two-beam
experiments, the optimal measurement is always provided by an observable
involving the detector only, in multibeam experiments, with equally populated
beams and two-state detectors, this is the case only for the Durr criterion, as
the other two require the introduction of an ancillary quantum system, as part
of the read-out apparatus.
