I never do this as we always have a new one on the shelf, fit it, and if the fault goes away it stays on and the old sensor goes in the bin, if it doesn't cure the fault, the old one goes back on and we look elsewhere for a problem. However, by the book, you take the sensor off and connect an ohmmeter to where blue/yellow had been (positive) and negative to where the green/yellow wire went, "rotate the fitting inside the sensor with a screwdriver and watch the meter, it should vary smoothly between 0 and 1.6 to 2.4 k-ohms." Remember, when putting it back, it is pre-loaded on the autos, fit it on the shaft then turn clockwise until the mounting holes are in line.