How can you know what happened millions of years ago if no one was there to see it?

Why did the carnivore dinosaur brains were larger than the herbivore dinosaur brains? What parts of the world was velociraptors found? What part of speech is dragon? Do pterosaurs still exist? Was the megladon the largest water dinosaur? How do you find Diglett's missing dad in Pokemon mystery dungeon explorers of time? What is the fastest predator during dinosaur time? Why do you think they are called terrible and marvelous Dinosaur? How tall is a Troodon? The cave where humans lived more than 9000 years ago? What are the differences between dinosaurs and human beings? When was the first triceratops fossil discovered? What is a diona? What is the muzzle velocity of a 557 t-rex bolt action riffle? Why is Cardenas shown standing on a pile of oil barrels? In Brisingr what is Ormis' dragons name? Where to get Cranidos in Pokemon Mystery Dungeon Explorers of Time and Darkness? Who or what is the rufinator? How big is the pterodactyls wing? What is spinosaurus physical adaptation?