If your baked goods are taking a significantly longer or shorter time to bake than they should, that is the first clue. Purchase or borrow a high-quality oven thermometer. Place the thermometer in the center of the oven’s middle rack. Turn the oven to 350 degrees F and preheat the oven. If the temperature on the oven thermometer reads more or less than 350 degrees F, your oven is out of calibration.