Some manufacturers say that using an oven thermometer (available at most hardware stores) isn’t accurate because once the door opens, the temperature changes, I think it’s probably a better measure than trying to decipher how far off your oven is by experimenting on baked goods or other methods.
First, check your thermometer’s accuracy by sticking it in boiling water for a minute. Boiling temperature is 212 degrees F, so if that’s not what your thermometer reads, note the difference.
Next, put your thermometer in the oven and select 350 degrees. Check the thermometer after about 20 minutes. If it doesn’t read 350 degrees (after factoring any difference you found in step one), you know whether your oven temperature runs high or low and how many degrees.
If you were smart enough — or organized enough — to save your Use & Care manual, your manufacturer might include instructions on how to calibrate your oven so that the temperature settings can be adjusted for accuracy. Otherwise, just make a mental note and select a 355 degrees (for instance) next time a recipe calls for 350.
Many ovens now come with built-in temperature probes, which offer another great way to ensure your meat cooks perfectly.
And the best way to ensure consistent temperature in your oven is to minimize opening the door by using your oven light. Having the oven door open for just seconds can decrease the temperature by 25 degrees!