Forward converter input current
With calculations like that you can get close, but usually there are too many parasitic features that need to be taken into account. If you want to know the efficiency for sure, you have to measure it.

Just measure input and output currents and voltages. You efficiency is η = (U_out * I_out) / (U_in * I_in)

You have to measure it with different loads because efficiency changes depending on the load.

Edit:
If you're just designing the converter, use a simulator to calculate the efficiency.