Just use these 2 rules of thumbs:
1) Craters provide cover in all directions.
2) For the rest, what you see is what you get? Is a model out of cover? Then that model does not get a cover bonus. Do the enemy bullets cross the cover you are standing behind? No? Well that means you are flanked and you get no cover bonus.
The only way to really know if the "bullets cross the cover" is if youre men die or stay alive. I think that this is a problem and that its related to what people call rng. I guess there are times when you think you are in cover when you are not. These times youre men will die in a irrational way because you think you are in cover. As I do feel my men were in cover I will blame their death on rng.