This paper introduces a technique to calculate the induced voltage generated by coupled-receivers and foreign-objects on the transmit-coil in real-time. Changes in the position or electrical quantities of the receivers, and foreign-objects, alter the induced voltage on the transmit-coil, and with it the trajectory of the switching-waveforms of the inverter driving the transmit-coil. From the shape of these waveforms, information on the phase and amplitude of the induced voltage can be extracted, thus enabling the induced voltage on the primary to be estimated with a single, easy to access, voltage measurement, which is easier than estimating the induced voltage from measurements of coil current and total coil-voltage. We used a support-vector-machine(SVM) to perform regression analysis on the drain-voltage data. The experimental setup uses a 100W, 13.56MHz Class-EF inverter, and the model was generated from a large number of samples of the drain-voltage waveforms operating at different known loads. These were generated from our in house HF-IPT test-load, which uses a Class-EF synchronous rectifier. The results allow the induced voltage on the transmit-coil to be estimated in real time from the drain-voltage waveform alone, with a normalised root-mean-square error of 1.1% for the real part~(reflected resistance) and 1.2% for the imaginary part~(reflected reactance).