There are teams of people who go out to dig up battlefield graves. Soldiers know this. So, if they have to bury someone, they generally mark the site. Bodies are also laid out with a degree of dignity, not tossed. In very simple terms, soldiers in modern conflict tend to die face-down. A battlefield grave will have the body face-up, covered in loose earth, and positioned like they were lying in a flat bed. A body crumpled up in a pit is almost certainly not a battlefield grave.

(Soldiers dont bury bodies while under fire. The burials happen after the battle is over by whatever side now holds the ground.)