As far as I know in a car engine the ECU can adjust fuel mass flow through every injector. I don't know the exact algorithm but probably engine roughness is monitored. Of course this is not possible in a jet engine. What will happen if an injector starts to malfunction? I assume it will be quite difficult to detect and a slight decrease in maximum power will appear. But engines usually are not run at full power? Can this be dangerous when maximum performance is needed (like engine failure at V1)? How are injectors tested in engines with mechanical fuel control? Can FADEC monitor fuel injectors?
Fuel nozzles are overhauled and bench tested to meet the manufacture's operational limits. They are controlled for both flow (back pressure) and also flow pattern. The FADAC monitors fuel flow, this along with the compressor vane position are the main input controls for thrust generation.
The fuel nozzles do degrade during engine's operation. This contributes to the engine running hotter as it ages. Other contributors are the erosion of the compressor hardware and the burning/oxidation of the combustion chamber and turbine hardware. In fact a defective fuel nozzles may lead to a "hot spot" that accelerates the burning/oxidation of the combustion chamber. The FADAC and flight crew will monitor the throttle response and also the Exhaust Gas Temperature (EGT) as the controls to understand the relative health of the fuel nozzles. If the engine exceeds the EGT limit a mandatory boroscope inspection is required to keep the Aircraft in service, this inspection will visually verify the condition of the fuel nozzles and combustion chamber.
Yes a FADEC can monitor fuel flow.
Let's assume you are operating a simple turbine. Normally you will have fuel flow, turbine speed and if a turboprop a torque monitor. The pilot monitors each of the steam gauges. The fuel control unit will normally limit max fuel flow.
An ECU is more akin to a FADEC, and the range of programmatic control in a FADEC is diverse, so a generalization is not as easy as in a simple turbine.
So to summarize, Yes, a FADEC can monitor fuel flow (and everyone I am familiar with does). And in the case of a more simple turbine application, the pilot, and the fuel controller (which will vary in implementation) can also provide limits.