Really ?
Am not sure about that... Do you understand an ampermeter at the pump motor to recognize the difference in current when water is present or not ?
Do you want to protect the pump that you do not have ?
Before I quit this discussion; slightly open a faucet in that questioned pipe. If water flows, drops falling on a sensor will tell presence; then enable signalling for the pump you do not have.
This has to be another suctioning-from-watermains scheme of the third world, right ?