What confirms the fact that the transfer of energy from the primary to the secondary of a transformer is not perfect?

In a theoretically perfect transformer, 100% of the electrical energy fed into the primary coil would be magnetically transferred to the secondary coil. In reality, no transformer is perfect; some energy is inevitably lost in the process and converted into heat. This heat comes from 'eddy currents' (stray currents circulating within the core itself) and 'hysteresis' (friction from continuously reversing the magnetic domains in the iron). The physical result of these energy losses is that the iron laminations of the transformer core become warm to the touch. Voltages and currents are simply operating parameters, not direct proof of energy loss.