Hi, Ross

Depending on the voltage applied, the trick could be simple:

for DC supply, just apply Heating power by PWM ... and allow a continuous current flowing into your resistor.

So simple ... just measure the voltage across your heating element during the "0v" part of the PWM ...

For AC supply, Low voltage ... the PWM will be replaced by burst mode ...and constant DC current only applied when no AC provided.

AC, High voltage ... a probe really seems the simplest way !!!

Alain