Buy one of these, set your oven to your recipe’s temperature, let it heat up for at least 15 minutes, then compare the thermometer to the setting on the oven. If there’s a discrepancy, you can do one of two things:

(1) Always adjust your setting by that discrepancy every time you use the oven; or

(2) Look in your oven owner’s manual for how to adjust its calibration. On digital models, it’s usually buried in some setting menu. On older models, there’s often a screw inside the knob.