Water in the concrete

Dears all

I am looking for a solution to detect the water added to the concrete in the concrete mixers trucks, and that should be linked to a tracking device to sent this kind of readings to Wialon platform.
Let's have a scenario where there is a concrete mixer truck ready in the site, and the workers added water inside the mixer, the sensor should detect this type of action and that will reflect on Wialon though the tracking device.

Is there a sensor in the market can help me in this case ?

Thank you