  Control systems are exposed to unintentional errors, deliberate intrusions,
false data injection attacks, and various other disruptions. In this paper we
propose, justify, and illustrate a rule of thumb for detecting, or confirming
the absence of, such disruptions. To facilitate the use of the rule, we
rigorously discuss background results that delineate the boundaries of the
rule's applicability. We also discuss ways to further widen the applicability
of the proposed intrusion-detection methodology.
