Monitoring 12dvc motor speed

Differential pressure sensors and a narrow outlet will not work for liquids as they are incompressible- only for gasses. That's one of the things I learned at work. They also told me if I could figure out a way to measure both high and low liquid flow rates accurately in a small in-line device I'd be rich.

I have a circuit here;
https://www.romanblack.com/current.htm
that uses one transistor to measure motor over-current and light a LED.
It will also drive a 0-5v signal line you can use to send to a PIC etc.

This is a circuit I have used for years for the same need you have, ie to sense motor jamming or bearing failure.