Cheap 5/8" poly irrigation tubing costs 5 cents/foot but degrades in the sun and unexpectably rupture creating a mess.
On solution would be to have venturis at both ends of the questionable tubing. Assuming the pressure drop across the length of tubing was low, the pressure in the throats of the venturis should be about the same if the water is flowing or if it is off.
Only when the tubing ruptures are the pressures different.
The valve could be double acting. In case of a small leak the down stream venturi would be higher and close the valve.
In case of a really large leak the pressure downstream might go to zero and the diaphram would seat the valve in the other direction.
Is there a common off the shelf diaphram or other valve that is normally open except when there's a pressure differential?