You cannot directly measure the resistance of an actively powered element. You can measure other values (voltage across the elements, current through the circuit) and calculate the effective impedance. If the voltage is always constant (100V) you can skip that part and assume a nominal value and just measure current to know if too much / too little is flowing which means the element is out of range.
This is effectively what a multimeter does, it uses its own internal power to inject a small current to measure the circuit parameters to determine the resistance. Any external power completely negates those conditions.
