How to identify source and load in AC network?
Hi: I guess it is really up to you,  you could decide what to call them arbitrarily. 

Or, you could compute the average power flow into each box.  If the power in is positive, the box is absorbing power, you could call that the load.  If the power in is negative, the box is supplying power. You could call that the source.  All physically existing circuits should obey conservation of energy, so the sum of the two average powers must be zero.