The question refers to non-compressible fluids.
I'm mostly thinking about monopumps (excentric screw pumps), but this also should apply to any type of gear pumps as well, possibly also to piston pumps.
Theoretically, it should be possible to measure the flow by simply counting the revolutions and multiplying by displacement volume per revolution.
The one source of error (I can think of) is the flux those pumps experience when operating against high heads and/or after some wear near the sealing line.
Are there other factors to be taken into consideration?
Are there real world examples?