Buy a portable oven thermometer and check it against the built-in thermometer in your oven. You'll have to check the portable thermometer first to make sure it's accurate, though. First, put the portable thermometer in boiling water and make sure it registers 212 degrees Fahrenheit. Or place it in water filled with ice cubes and verify that it reads 32 degrees Fahrenheit. You can use those temperatures as a baseline to establish the accuracy of the test thermometer you're using.