How to determine which triangle is DWL?
Here's a quick and dirty trick: 


  DWL is the triangle that points (horizontally) towards the efficient quantity. 


If equilibrium quantity is lower than the efficient quantity, DWL should point rightward, and it represents the amount of unrealized gains from trade. 

If equilibrium quantity is higher than the efficient quantity, DWL should point leftward, and it represents the amount of social loss due to over-production/consumption (when social marginal cost is greater than social marginal benefit).