The best way for you will probably be to use an oven thermometer. Place it in the middle of the baking tray as you would a cake/batch of cookies etc. Next, set the oven temperature to the supposed baking temperature and leave it that way for at least 20min to simulate the preheating. After this, you can start recording the temperature displayed on the thermometer in 5min intervals over an hour perhaps to detect fluctuation patterns or steady deviances etc. If need, repeat the process with different temperature settings on the oven to find out exactly how your oven performs. If it fluctuates haphazardly or heats different areas very unevenly, perhaps it will be time to get a new oven ^^"