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

Did a brachiosaurus live alone or in a herd? When did pterodactyl die? How tall are the dinosaurs from walking with the dinosaurs? When the plesiosaur died? How did Barosaurus dinosaurs die? Are chickens related to t Rex's? What is the estimated life span of the huge sauropod dinosaurs? How much do triceratops eggs weigh? Was the apatosaurus a producer? How big is the tongue of a t rex? Who discovered the pterodactyl? What squeamish people won't eat? What kind of teeth do pterodactyls have? What is a record of past living things called? Why is the rainforest valuable? How do you use the word din in a sentence? What is name of dragon in Mulan? What does the T Rex's Humerus Bone Looks Like? Why is Steve so bald? How did mesosaurus and lystrosaurus provide evidence of Pangaea's existence?