This schematic looks almost identical to the power supply in a '70s era CD electronic ignition or electronic flash for photography.
Determination of the oscillating frequency is not easily calculable. The frequency of operation is dependent on the saturation of the magnetic core. One transistor turns on. Its base gets positive feedback from the second winding, shown drawn on the primary side. Current rises with time until it becomes so great that the core saturates, this second winding loses voltage, the transistor turns off, and inductive flyback turns on the other transistor. Repeat ad infinitum.
Determination of the operating frequency is best done by experimentation..... unless someone else has a better idea?