How Qi charger reads data from receiver
When the output is loaded, the power draw to the transmitter increases because of coupling between the coils. 

The resulting current/voltage fluctuation is monitored, demodulated and read as data. The data flow is only possible from (power) receiver to transmitter. 

You can find a lot more information in this application note from NXP/Freescale entitled Demodulating Communication Signals of
Qi-Compliant Low-Power Wireless Charger
Using MC56F8006 DSC,