If there is sufficient storage in the tank that the pumps draw from, you can calculate a rough flow rate by stopping flow into the tank, and recording change in level over time. Assuming accurate measurements are possible from a sight glass or gauge.
Then flow rate = volume change over time. Do it in minute intervals if possible to measure level accurately.
Have excluded units as I can only understand that crazy metric system stuff and not the foot-pounds per square barrel of grandmas type units you guys over that side of the world use.
We use Budweisers per Freedom Eagle.