The method detects the presence of an effective corrosion inhibitor in a central heating system liquid. A clean steel electrode and a non-steel electrode are immersed in the system liquid. The galvanic current flowing between the electrodes is measured after a delay, and the measured current is compared with a reference. A portable apparatus for carrying out the method comprises a sample container (10) and a measurement instrument (12) which includes a steel electrode (14), a non-steel electrode (16), circuitry (19) for detecting galvanic current flowing between the electrodes when immersed in a test liquid in the container, and generating a measurement signal indicative of the measured galvanic current. The method and apparatus provide a quick effective method of checking correct inhibitor function.