Geologists have been studying and codifying various layers of rock and the types of fossils within them across the world for centuries. So it’s well established how old the exposed rock layers are in any particular part of the world long before they had any idea how old things are. Then as dating methods have improved we can make correspondences between these rocks and their age. So as mentioned by another commenter, paleontologists can take that knowledge of how old the rocks are, and what environment they were laid down in, and how they were formed, along with our existing knowledge of the age of various species, and their locations, and then find a likely spot with easily accessible layers from the right location and time period and conditions and they might get lucky.