The sensor is a eBay bought sensor with an LM393 IC and got 4 connections DO VCC GND AO
Post a link to the sensor (NOT the stupid EBAY link).
Reading a hall effect sensor is relatively easy. Having the sensor trigger an interrupt is another possibility.
Deciding what "when the flow stops" means is a different story. You'll need to keep track of when you get a signal, and decide how long between signals means that the flow has stopped (or dropped to too low a rate).