The very first thing to do when you are testing a furnace thermostat is to replace the battery if it is battery-operated. If you have any questions about the performance of your thermostat, and it has been a while since you replaced the batteries, then replace them before doing anything else. To do a thorough test of your thermostat, you will need two people. One person needs to be near a furnace vent, and it needs to be absolutely quiet in the house. The second person begins adjusting the thermostat up. If the thermostat and furnace are working, the person listening to the furnace will hear clicking noises. If there are no clicking noises, then the furnace thermostat is not working.