[strictlypersonl]

I'm not sure (old-timer's disease) but on the very early cars, we may have used a BMW reservoir cap with a float. Mid-production cars used two special brake light switches that signaled when there was pressure in one end but not the other. We gave up on those when their quality went down the tubes.
Now, you just keep track of whether the clutch release is working properly. If the fluid drops too low, that will be the first indication of a problem, either in the clutch or the brakes.