Ovens often have a little tell-tale neon light to show that the element is taking current (i.e. Power still being fed to element because oven not yet up to set temperature).
When the light goes out it suggests that the oven has reached the set temperature.
Unless your neon is broken - or your oven does not have the luxury of this light - you can use it as a diagnostic aid.
Set the oven to a medium heat and watch the lamp - when it goes out open the door and sense if the temperature inside matches the setting on the temperature control.
If the oven is on the cool side then the thermostat may be switching off prematurely - if the light never goes out it suggests the element (or one of the element sections) is not working properly (or at all).
Peccavi
April 2009