Kirchhoff's Laws and Circuits?
When analyzing a circuit, you can put the arrows in either direction according to whim, a flipped coin, or Tarot cards.  

After applying Kirchoff's laws to compute all the voltages and currents, you'll find some variables have negative values.  Those correspond to arrows you drew backwards.  Fix those, and then you know the directions of currents in all branches of the circuit. 

It is perfectly normal for an experienced engineer to get a few initially backwards, when multiple different voltage sources are pushing in opposite directions. You can only guess, and let algebra tell you the net result.