1.) An anode rod protects the water heater lining against corrosion and prolongs the life of the water heater. If you have hard water, check yours once a year or so and replace it if it is encrusted.
2.) Anode rods with heavy corrosion will cause an rotten egg or sulfur smell when running the hot water. This is a sign that the anode rod needs to be replaced and the water heater flushed.
3.) See manufacturer recommendations on how to check and replace your anode rod.