  We study the combined counting statistics of two capacitively coupled
transport channels. In particular, we examine the conditions necessary for
utilizing one channel as detector sensitive to the occupation of the other. A
good detector fidelity may be achieved in a bistable regime when the tunneling
rates through the two channels are vastly different -- even when the physical
back-action of the detector on the probed channel is large. Our methods allow
to estimate the error of charge counting detectors from time-resolved current
measurements -- which have been obtained in recent experiments -- alone.
