I suppose what I am asking is how does one spot this accurately during a normal "A" and power check?
After a certain time flying a certain aircraft, you know the sound an engine makes at a certain RPM. That will give you a rough indication.
If it's a recent failure of the tacho, then the idle RPMs will be very abnormal, while the engine idle settings have not changed. Likewise, when starting the take-off roll, there are minimum and maximum RPMs that you should see when applying full power.
Another way is by setting cruise RPM and not getting the associated cruise performance (IAS). But then again that might be caused by a faulty pitot/static system or faulty ASI too, so double-check with DME/GPS and correct for wind.
But indeed, those are general indications and not very accurate.