How can you be sure your oven is holding the right temperature?

Most oven thermometers I have tested are unreliable. The best way is by baking a reliable recipe. If the recipe says bake 30 to 40 minutes and it is done in 25, turn it down 25 degrees. If it takes longer than 40 minutes turn it up 25 degrees. Occasionally oven thermostats become erratic and do not hold temperatures no matter what the setting. This requires professional calibration or a new thermostat.