  We address the problem of unambiguous discrimination and identification among
quantum observables. We set a general framework and investigate in details the
case of qubit observables. In particular, we show that perfect discrimination
with two shots is possible only for sharp qubit observables (e.g. Stern-Gerlach
apparatuses) associated with mutually orthogonal directions. We also show that
for sharp qubit observables associated to nonorthogonal directions unambiguous
discrimination with an inconclusive result is always possible.
