How to check for functional asymmetry?

• When the consumption in the building is switched off, it is necessary to switch on a single-phase appliance on each of the three phases. The inverter shall maintain power at 0 W at the house meter foot for each of the three phases. 
• If the smartmeter shows a draw on one phase and an export to the grid on the other, it will indicate a swap of these phases and a repair is required. 
• Without this check, the malfunctioning asymmetry will show up on the energy supplier's bill.