  We introduce a new decentralized observation condition which we call "at
least one can tell" (OCT) and which attempts to capture the idea that for any
possible behavior that a system can generate, at least one decentralized
observation agent can tell whether that behavior was "good" or "bad", for given
formal specifications of "good" and "bad". We provide several equivalent
formulations of the OCT condition, and we relate it to (and show that it is
different from) previously introduced joint observability. In fact, contrary to
joint observability which is undecidable, we show that the OCT condition is
decidable. We also show that when the condition holds, finite-state
decentralized observers exist.
