Sometimes the fossils are complete enough to see remnants of stomach contents. However, the best clues/evidence comes form the shape of their mouth and the structure of the teeth. Their body morphology also gives clues.
The dinosaur ate his prey slowly.
velocirapter
carnivores
sientist can not even figure it out yet so go ask your parents
It was celophysys a small dinosaur that hunted in groups and ate fish.
I seriously doubt it.
a dinosaur that ate a chicken
Parasaurolophus ate plants from trees.
it ate meat
It was the t Rex and then the folaserrapter
Plateosaurus was an herbivore, which means that they ate plants.
There are many ways to tell what a dinosaur ate. One way is looking at it's teeth. Blunt spoon-shaped cheek teeth indicate the dinosaur ate plants. Sharp pointed teeth show that dinosaur ate meat. Another way is looking at the skeletons. Sometimes you will find the skeleton of the prey inside the skeleton of the predator. And yet another way is looking at it's droppings.