In general you will distinguish an abnormal man made looking stretched out or rounded "built-up area (breastwork)" parallel and adjacent to a depressed area called "trenches" both having resulted from digging with the "built-up" area closest to enemy positions. Some are more difficult to make a distinction instantaneously and some are more predominate than others due to weathering, soil, and battle conditions and can have trees growing out of them. The wide circular ones are called sometimes called "gunnery pits" which housed artillery. Hope this helps