How do you measure fluid flow?
We measure the movement of mass. It is basically the conservation of energy in a system.
There are very many different physical ways to measure fluid flow, based on different parts of the Bernoulli equation. One of the simplest in construction, if not calculation, is the pressure drop across on orifice.
The change in pressure from one side of an orifice plate to the other in a fluid is proportional to the square root of the fluid mass flow.
see also: https://www.grc.nasa.gov/WWW/K-12/airplane/bern.html