How to determine which rail is under load
The current flowing will be zero for the circuit you have posted.  

When the voltages are different, ideally the current will be the same flowing through the supply V1 and V2.  

In practice, the current also depends on whether the supply voltage can also sink current. If not, either the supply will be damaged or there wont be  any current. 

Edit 2: 

After realising the negative sign in the bottom supply the voltage source both are sharing the load equally. The push force by the +15V is same as the pull force by the -15 V supply. They add up to be a equivalent +30 V push or -30 V pull force depending on current direction we assume.