How to determine whether oven thermometer is accurate?
Use a pot of water!


Find out the lowest temperature of your oven (needs to be at least below 212 F for it to work). Preheat the oven to this heat
Heat water on the stove to a temperature let's say, about 10 F below the oven temp. Use a candy thermometer and leave it in the water, but not touching the bottom of the pan.
Once the oven is preheated, transfer the pot of water to the oven, and leave the candy thermometer on the whole time.
Periodically check the temperature of the water. It should asymptotically approach the temperature of the oven. If the water temp closes in on the temp of the oven, the oven thermometer is calibrated!
You can verify by checking the temperature on the opposite end. Warm the water on the stove to 10 F above the oven temp, and watch the candy thermometer go down to the oven temp.


Note: it is very important to get the water on the stove close to the oven temp. Water has a very large specific heat capacity and will take quite a while to heat up in the oven, as it takes a large amount of energy to heat water.