Finding the end of resonance in quasi-resonant zero voltage switching inverter (attached figure)

I would be grateful if anyone could help me for suggesting a system/or circuit that determines the time when CE voltage of Q701 equals to zero and also voltage of primary transformer reached to the line voltage. For starting the switch softly.

In the realization of this circuit there are four feedback branches from Vce and one from upper terminal of transformer (as the figure).

Thank you so much for spending your valuable time on reading this mystery.