Unplug the power, and test for continuity to look for fault in:
* heating element
* wiring
* connections
It may be risky to hook up power directly to the element. You should try it, but you ought to disconnect it from sensitive circuitry. If the element heats up then you almost certainly have a bad control unit.
The control unit may contain components that are difficult to test. Maybe it's a triac that's bad. Or a thermostatic sensor. Do you know how these components work? Then perhaps you can test them individually. Or else install known good components.