I don’t think scientists do know that ALL dinosaurs laid eggs do they? I’m pretty sure I remember reading something about scientists finding evidence that some marine dinosaur gave birth to live young. But I think they agree the majority of dinosaurs laid eggs. Aside from the fossil record there’s also the genetic evidence. The modern day animals that dinosaurs are mostly closely related to all lay eggs (like birds and reptiles). https://www.amnh.org/dinosaurs/dinosaur-eggs