Reverse flow rate can be detected with the use of a differential pressure instrument. (Cost effective way to detect reverse flow)

Reverse flow condition can be detected with the use of a differential pressure instrument. When differential pressure is measured across a restriction in the flow line , we can measure flow rate. In applications where change in direction of flow is to be detected,  a center zero instrument can indicate flow direction as well as measure flow. If the fluid in the line is not flowing, the pointer will remain at zero position and will deflect in either direction depending on the direction of the flow. Additionally, a switch can be added to alarm for a reverse flow condition.