How to see if you are behind an entity

My objective is to do more damage if you are behind an entity. I have tried a lot of things with .getDirection() to check to see if one entity is facing the same direction as another entity. My basic principle of thought is this: If you damage an entity and you're facing almost the same direction as the entity, you must be attacking the entity from behind. If anyone could help me with the methods/math involved in this, I would greatly appreciate it!