How does coprolite help us determine if a dinosaur was a carnivore or herbivore?

The food remnants in the coprolite would tell whether the animal was a carnivore, herbivore, or omnivore. A carnivore's dung would not contain plant matter and would likely have bone fragments. An herbivore's dung would probably have some amount of undigested plant fibers left in it. An omnivore's would probably have a combination of plant and animal remnants, unless it had only eaten one of those recently.