I grazed over it in the
Flow Measurement thread.
One option is to go digital. As I painfully discovered the limits of my flowmeter
here, I also came up with a way to digitally measure pressure. Relatively cheap, but I never pursued it very far, other than the possibility, and I didn't delve into the accuracy, repeatability, nor calibration.
Otherwise, the simplest way is to use a manometer, as I've suggested many times before, for heatercores: