How do you mean? Dinosaurs? They were warmblooded, so no, not just lizards. Usually in the rock around the fossilized bones are imprints of other features of the animal, like skin texture or the presence of feathers.

We can also tell a lot from the points where muscles attach to the bones.

That's not even accounting for the behavior we can interpret based on fossilized footprints! We're actually learning that T-Rex were pretty good parents for instance.

It's a very complex field, and it's amazing how much we can learn from so few clues.