How do we actually know, how the dinosaurs look like? At the time of the dinosaurs there wasn't anybody who could have written something down for us.
Up to these days bones or strange tracks are discovered worldwide over and over again. These remains, the so-called fossils, tell scientists a lot about the appearance and the life-style of the animals. With a lot of luck a bone is still so well-preserved that you don't even know how old it really is. Unfortunately, such findings are only extremely seldom.
Far more often fossilized bones are found. In this case sand got in the gaps and the holes of the bone, then became hard and so fossilized the bones over the time. These fossilized bones also help to find out how the animal has looked liked.
However, it can also happen that holes are found in the ground which look like as if a bone had lain there a long time ago. The scientists think that the bone itself was enclosed by hard stone and has simply resolved then. If sand penetrates into such holes, a bone of stone forms from it after a long time. If the hole remains empty, only the impression of the bone can be found. Those leftovers help the scientists to get a picture of the animal the bone could have belonged once.
In rare cases also impressions of the skin are found which help us to get a clue about whether the animal had scales, a fur or even feathers. Every now and then even animal tracks like footprints and eggshells or even whole eggs are discovered. With the help of those tracks we can find out how the animal has lived, what it has eaten, how fast it could run, whether it lived alone or in a herd, if it has looked after its young ones and so on.