Years ago my father worked for Eastern Airlines out of Miami (that should give you a timeframe) and he was an electrical engineer. I recall once he told me that the pilots (one or more) used to intentionally damage the CVR microphone in the cockpit by twisting it until the contacts broke. Their reasons are not relevant for this question.
My question is, knowing what model planes they flew back then, (727, Tristars, A300s, 757, etc) how did the mechanics know that the mic was not working? Would the system show it as a fault or did the CVR tape review show that there was no data from that mic? The former I'd think would allow them to track down the pilot(s) that were responsible for this act.
Also, on planes with more modern electronics, would this fault be detectable immediately or at least before the next flight?