Measure ~300kHz Induction Heater RF Power
I would think measuring the magnetic field of the induction heater with another coil might be the way to go. With an inductor the primary filed is magnetic. Since magnetic fields fall off with the distance cubed (1/R^3) it would be difficult to pick up other sources of magnetic noise at 300kHz. You could find a coil with a response at 300kHz or a magnetic sensor with a response at 300kHz.