I'm not entirely sure a rev limiter will help protect you if you downshift at a speed higher than the gear you've selected is intended for. If you manage to lift the clutch, even without any fuel, throttle or spark, you could do some fairly significant damage.
To answer the question regarding the rev limiter. If a carburetor or mechanical fuel injection equipped car has a rev limited, it will take the form of a mechanical mechanism built into the rotor arm.
Typically rotor arms are fairly simple and compact in that they'll have a straight copper connector running along a straight arm and little else. If a rev limiter is built into the rotor arm they'll typically take on a more complex appearance. You may see springs built into them or they'll look more like a cam or snail shell.
Now, advice regarding downshifting. Take some time to learn the speed readout at the maximum RPM of each gear and simply adapt your driving style so that you don't downshift to a gear when the speed of the vehicle is close to or exceeds the known maximum for this gear.
For example, my Golf is bouncing off the rev limiter at 30mph in first. I therefore don't select first unless the speed of the car drops well below 30mph. With second it's around 55mph and third is approaching 80mph.