I know when I have hit an Enemy GV because of the distinctive hit graphics showing a small explosion on the GV.
I also know when I hit the trees either near me or near the enemy as the flash seems a lot bigger, especially when its in the tress/bushes by your own position.
Hits on the ground seem clear enough with the round usually making a small crater and kicking up earth/dust
The one that gets me confused is the smallest flash. Smaller than a normal hit with no indication of it being long or short either! Is this a hit that bounces off armour without causing normal hit damage ?