How do I know if my digital thermometer is accurate? Add a little clean water until the glass is full and stir. Wait for about three minutes before inserting the sensor on the thermometer into the ice-filled water. Wait for about thirty seconds and check that the thermometer reads 32°F. If it does, then it is accurate, but if not, it requires calibration. This is by far the most accurate method, and it will also give you an idea of how far off your readings are when it comes time to reset it.