Checking the Temperature. Put an oven thermometer at the center of the oven and set the oven to 350 degrees F. Check the temperature indicated on the thermometer after 20 minutes. If it's not within 5 degrees of 350, you'll need to adjust the oven's temperature control.ut an oven thermometer at the center of the oven and set the oven to 350 degrees F. Check the temperature indicated on the thermometer after 20 minutes. If it's not within 5 degrees of 350, you'll need to adjust the oven's temperature control.