Distilled water will have a conductivity in the range 0.5 to 3 micro-siemens per cm.
Rivers in Australia will generally lie in the range 30 to 2000 micro-siemens per cm.
Sea water is about 51,500 micro-siemens per cm.
Conductivity is measured using an electronic device. The measurement is very sensitive to temperature change. The best meters measure and compensate for temperature automatically, saving some messing about with a thermometer and calculator.
I use a Eutech Cybernetics 'TDScan 20' supplied by WaterWatch. This includes automatic temperature compensation.
It can take 30 to 60 seconds to gain a reasonably stable reading as it adjusts to temperature variation.
It requires 4 small button cell batteries.